[clang-tools-extra] [libunwind] [llvm] [clang] [libunwind] Replace process_vm_readv with SYS_rt_sigprocmask (PR #74791)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> Hmm. It looks like `[[maybe_unused]]` is still disallowed...

`[[maybe_unused]]` should be allowed when the pull request includes commit 47413bb2760e63a3302871ea770d6c0f5a742036 (C++17 for libunwind).

https://github.com/llvm/llvm-project/pull/74791