SafeStack: Fix thread liveness check on *BSD
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

git-svn-id: c199f293c43da69278bea8e88f92242bf3aa95f7@340856 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt,llvm-mirror/compiler-rt