[X86] Extend some Linux special cases to cover kFreeBSD.

Both Linux and kFreeBSD use glibc, so follow similiar code paths.
Add isTargetGlibc to check for this, and use it instead of isTargetLinux
in a few places.

Fixes PR22248 for kFreeBSD.

Differential Revision: http://reviews.llvm.org/D19104

llvm-svn: 268624