[libunwind] [libunwind] Replace process_vm_readv with SYS_rt_sigprocmask (PR #74791)

[Jordan R AW via cfe-commits]

ajordanr-google wrote:

Rebased to include 47413bb2760e63a3302871ea770d6c0f5a742036 (a merge commit seemed silly?). No other changes have been made.

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