SafeStack: Fix thread liveness check on *BSD

Summary:
The Linux/BSD system call interfaces report errors differently, use the
internal_iserror() function to correctly check errors on either.

Reviewers: eugenis

Reviewed By: eugenis

Subscribers: delcypher, llvm-commits, #sanitizers, krytarowski, kcc, devnexen

Differential Revision: https://reviews.llvm.org/D51368

llvm-svn: 340856