Merge bitcoin/bitcoin#27699: random: drop syscall wrapper usage for getrandom()

5228223e1ff2af29e6e77668ce3288005c2adbbc ci: remove MSAN getrandom syscall workaround (fanquake)
d5e06919db5e221bfef445c5a40c88de72dc5869 random: switch to using getrandom() directly (fanquake)
c2ba3f5b0c7d0eece7d16d1ffc125d8a6a9297af random: add [[maybe_unused]] to GetDevURandom (fanquake)
c13c97dbf846cf0e6a5581ac414ef96a215b0dc6 random: getentropy on macOS does not need unistd.h (fanquake)

Pull request description:

  This requires a linux kernel of `3.17`+, which seems entirely
  reasonable. `3.17` went EOL in 2015, and the last supported `3.x` kernel
  (`3.16`) went EOL > 4 years ago, in 2020. For reference, the current
  oldest maintained kernel is `4.14` (released 2017, going EOL Jan 2024).

  Support for `getrandom()` (and `getentropy()`) was added to
  glibc `2.25` https://sourceware.org/legacy-ml/libc-alpha/2017-02/msg00079.html:
  > * The getentropy and getrandom functions, and the <sys/random.h> header
    file have been added.

  and we already require `2.27` or later.

  All that being said, I don't think you would encounter a current day (+~6 months from now)
  system, running with kernel headers older than 3.17 (released 2014) but also having a
  glibc of 2.27+ (released 2018)?

  Removing this (our only) use of `syscall()` also means we can drop a workaround in our MSAN jobs.
  If this is merged, I'll drop the [same workaround in oss-fuzz](https://github.com/google/oss-fuzz/blob/25946a544856413d31d9cbb3a366a4aef5a8fd60/projects/bitcoin-core/build.sh#L49-L56).

ACKs for top commit:
  josibake:
    ACK https://github.com/bitcoin/bitcoin/pull/27699/commits/5228223e1ff2af29e6e77668ce3288005c2adbbc
  hebasto:
    ACK 5228223e1ff2af29e6e77668ce3288005c2adbbc, I've tested build system changes on Ubuntu 22.04 and macOS Monterey 12.6.6 (x86_64).

Tree-SHA512: cc978e08510c461b875ca8c08ae176b4519fa1108f0efd74dcb7474518945357e0184e54423282c9a496de195e4ddc3e221ee78623bd63e24c50cc86acdf32e2

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