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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23482 91177308-0d34-0410-b5e6-96231b3b80d8