[sanitizer] Implement internal_fork and internal_forkpty for OS X

On Linux, we have internal_fork that forks without invoking user's pthread_atfork handlers, which is important for spawning external symbolizers. Let's implement this for OS X as well (using __fork). This patch also adds internal_forkpty which re-implements forkpty and uses __fork in it as well.

Differential Revision: http://reviews.llvm.org/D14869

llvm-svn: 253666