Darwin, like many BSD systems, has a setjmp/longjmp which saves the signal mask

on setjmp calls and restores it on longjmp calls (both of which require syscalls).

This makes the calls REALLY slow.  Use _setjmp/_longjmp instead.  This speeds up
hexxagon from 120.31s to 15.68s: from 5.53x slower than GCC to 28% faster than GCC.

llvm-svn: 23482