[libc] Add sys/mman syscall wrappers (#195103)
Added ErrorOr-returning syscall wrappers for mmap, munmap, mprotect, and
pkey_mprotect in src/__support/OSUtil/linux/syscall_wrappers/. Migrated
the sys/mman Linux entrypoint implementations to use them, following the
design in libc/docs/dev/syscall_wrapper_refactor.rst.
Removed the shared mprotect_common.h in favour of per-syscall wrapper
headers. Added hdr/sys_mman_macros.h proxy header.
