Merge bitcoin/bitcoin#21793: build: use `-isysroot` over `--sysroot` on macOS

cf971c9ea0e8637d36a7ab8b7b8804d9b89f1d9a build: use -isysroot over --sysroot on macOS (fanquake)

Pull request description:

  Not only does this seem to be the more correct behaviour when targeting Darwin, but if you use `-isysroot`, Clangs Darwin driver will [infer the deployment target](https://github.com/llvm/llvm-project/blob/3e037f8f0e26acab8cc784ea4c7d05da79f7c22e/clang/lib/Driver/ToolChains/Darwin.cpp#L1652) from the SDK and use other SDK info when parsing arguments to the linker. In the case of [`-platform_version`](https://github.com/llvm/llvm-project/blob/3e037f8f0e26acab8cc784ea4c7d05da79f7c22e/clang/lib/Driver/ToolChains/Darwin.cpp#L2656), which is added if the linker is [new enough](https://github.com/llvm/llvm-project/blob/3e037f8f0e26acab8cc784ea4c7d05da79f7c22e/clang/lib/Driver/ToolChains/Darwin.cpp#L342), the version tuple is constructed from the SDKInfo, and SDKInfo, as far as I can tell, only exists when `-isysroot` has been passed, see [parseSDKSettings](https://github.com/llvm/llvm-project/blob/3e037f8f0e26acab8cc784ea4c7d05da79f7c22e/clang/lib/Driver/ToolChains/Darwin.cpp#L1785)

  As a result, the SDK version field in the `LC_BUILD_VERSION` command is filled out. i.e when building master:
  ```bash
        cmd LC_BUILD_VERSION
    cmdsize 32
   platform 1
      minos 10.14
        sdk n/a
     ntools 1
       tool 3
    version 650.9
  ```
  vs this PR:
  ```bash
        cmd LC_BUILD_VERSION
    cmdsize 32
   platform 1
      minos 10.14
        sdk 11.3
     ntools 1
       tool 3
    version 650.9
  ```

  This, from what I understand, will fix the issue we are having with Qt deciding wether or not to enable features like "Dark mode" on macOS, see #21771, however I have not tested that. Thus this is an alternative to #21782.

  Our usage of `--sysroot` was added in #17118.

  ```bash
  -isysroot <dir>         Set the system root directory (usually /)
  ```

ACKs for top commit:
  Sjors:
    tACK cf971c9ea0e8637d36a7ab8b7b8804d9b89f1d9a
  hebasto:
    re-ACK cf971c9ea0e8637d36a7ab8b7b8804d9b89f1d9a, only rebased and addressed comments since my [previous](https://github.com/bitcoin/bitcoin/pull/21793#pullrequestreview-647321518) review.

Tree-SHA512: f01138179fb85083b5505bbaa48810451098ffa4da5d3c9b673785448790aa76f2e64b2aab6e698f6ee378a21f70626445a3fabee7c61dbfc44e96f3e3964656

0 files changed, 0 insertions, 0 deletions