Merge #17738: build: remove linking librt for backwards compatibility

f7453dcc0386a4a1162ced1a490c096afa13178a build: remove linking librt for backwards compatibility (fanquake)

Pull request description:

  Now that we require glibc 2.17+, see #17538, we can remove linking librt
  for backwards compatibility purposes. The `clock_*` functions from librt
  were merged into glibc as part of the [2.17 release](https://sourceware.org/ml/libc-announce/2012/msg00001.html):

  * The `clock_*` suite of functions (declared in <time.h>) is now available
    directly in the main C library.  Previously it was necessary to link with
    -lrt to use these functions.  This change has the effect that a
    single-threaded program that uses a function such as `clock_gettime' (and
    is not linked with -lrt) will no longer implicitly load the pthreads
    library at runtime and so will not suffer the overheads associated with
    multi-thread support in other code such as the C++ runtime library.

  Note that `librt` is already unused by the RISC-V and AARCH64 binaries as their librts don't export any `clock_*` functions. As an example, you can find a diff of the arm32 vs arm64 librt symbols [here](https://gist.github.com/fanquake/b08cb1f0d14df3133395d7796ebf030c).

  Below is the library usage for the `v0.19.0.1` release (can delete these tables pre-merge).

  #### RISC-V
  ```bash
  riscv/bin/bitcoin-cli: ['libpthread.so.0', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-riscv64-lp64d.so.1']
  riscv/bin/bitcoin-qt: ['libpthread.so.0', 'libfontconfig.so.1', 'libfreetype.so.6', 'libxcb.so.1', 'libdl.so.2', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-riscv64-lp64d.so.1', 'libatomic.so.1']
  riscv/bin/bitcoin-wallet: ['libpthread.so.0', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-riscv64-lp64d.so.1', 'libatomic.so.1']
  riscv/bin/bitcoind: ['libpthread.so.0', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-riscv64-lp64d.so.1', 'libatomic.so.1']
  riscv/bin/bitcoin-tx: ['libpthread.so.0', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-riscv64-lp64d.so.1']
  riscv/bin/test_bitcoin: ['libpthread.so.0', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-riscv64-lp64d.so.1', 'libatomic.so.1']
  ```

  #### AARCH64
  ```bash
  aarch64/bin/bitcoin-cli: ['libpthread.so.0', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-aarch64.so.1']
  aarch64/bin/bitcoin-qt: ['libpthread.so.0', 'libfontconfig.so.1', 'libfreetype.so.6', 'libxcb.so.1', 'libdl.so.2', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-aarch64.so.1']
  aarch64/bin/bitcoin-wallet: ['libpthread.so.0', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-aarch64.so.1']
  aarch64/bin/bitcoind: ['libpthread.so.0', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-aarch64.so.1']
  aarch64/bin/bitcoin-tx: ['libpthread.so.0', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-aarch64.so.1']
  aarch64/bin/test_bitcoin: ['libpthread.so.0', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-aarch64.so.1']
  ```

  #### ARM LINUX GNUEABIHF
  ```bash
  arm32/bin/bitcoin-cli: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-armhf.so.3']
  arm32/bin/bitcoin-qt: ['libpthread.so.0', 'librt.so.1', 'libfontconfig.so.1', 'libfreetype.so.6', 'libxcb.so.1', 'libdl.so.2', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-armhf.so.3']
  arm32/bin/bitcoin-wallet: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-armhf.so.3']
  arm32/bin/bitcoind: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-armhf.so.3']
  arm32/bin/bitcoin-tx: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-armhf.so.3']
  arm32/bin/test_bitcoin: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-armhf.so.3']
  ```

  #### LINUX X86_64
  ```bash
  x86_64/bin/bitcoin-cli: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-x86-64.so.2']
  x86_64/bin/bitcoin-qt: ['libpthread.so.0', 'librt.so.1', 'libfontconfig.so.1', 'libfreetype.so.6', 'libxcb.so.1', 'libdl.so.2', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-x86-64.so.2']
  x86_64/bin/bitcoin-wallet: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-x86-64.so.2']
  x86_64/bin/bitcoind: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-x86-64.so.2']
  x86_64/bin/bitcoin-tx: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-x86-64.so.2']
  x86_64/bin/test_bitcoin: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux-x86-64.so.2']
  ```

  #### LINUX i686
  ```bash
  i686/bin/bitcoin-cli: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux.so.2']
  i686/bin/bitcoin-qt: ['libpthread.so.0', 'librt.so.1', 'libfontconfig.so.1', 'libfreetype.so.6', 'libxcb.so.1', 'libdl.so.2', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux.so.2']
  i686/bin/bitcoin-wallet: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux.so.2']
  i686/bin/bitcoind: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux.so.2']
  i686/bin/bitcoin-tx: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux.so.2']
  i686/bin/test_bitcoin: ['libpthread.so.0', 'librt.so.1', 'libm.so.6', 'libgcc_s.so.1', 'libc.so.6', 'ld-linux.so.2']
  ```

ACKs for top commit:
  laanwj:
    ACK f7453dcc0386a4a1162ced1a490c096afa13178a

Tree-SHA512: b418260edcda88583abfa386a592ebfb977d111e8e2ba887a30bf830b0b10dba429b9cfd615fad453ff0bb824225914ccb91433064b158ae1fbb9d20fc0b9937

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