[tsan] Use direct syscalls for internal_mmap and internal_munmap on OS X

On OS X, internal_mmap just uses mmap, which can invoke callbacks into libmalloc (e.g. when MallocStackLogging is enabled). This can subsequently call other intercepted functions, and this breaks our Darwin-specific ThreadState initialization. Let's use direct syscalls in internal_mmap and internal_munmap. Added a testcase.

Differential Revision: http://reviews.llvm.org/D18431

llvm-svn: 264259