[tsan] Fix build errors for TSan on OS X

This patch moves a few functions from `sanitizer_linux_libcdep.cc` to `sanitizer_posix_libcdep.cc` in order to use them on OS X as well. Plus a few more small build fixes.

This is part of an effort to port TSan to OS X, and it's one the very first steps. Don't expect TSan on OS X to actually work or pass tests at this point.

Differential Revision: http://reviews.llvm.org/D14235

llvm-svn: 251918