Enhance support for NetBSD in SafeStack

Summary:
Always try to detect and call internal or real libc symbols instead of
locally installed interceptors.

This covers:
 - GetTid()
 - TgKill()
 - Mmap()
 - Munmap()
 - Mprotect()

This cherry-picks code from sanitizer_common/sanitizer_netbsd.cc.

Reviewers: vitalybuka

Reviewed By: vitalybuka

Subscribers: llvm-commits, mgorny, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D57179

llvm-svn: 352154