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

[Alexander Richardson via cfe-commits]

https://github.com/arichardson approved this pull request.

Just one question otherwise this looks good to me.

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