Add __builtin_setjmp/_longjmp supprt in X86 backend

- Besides used in SjLj exception handling, __builtin_setjmp/__longjmp is also
  used as a light-weight replacement of setjmp/longjmp which are used to
  implementation continuation, user-level threading, and etc. The support added
  in this patch ONLY addresses this usage and is NOT intended to support SjLj
  exception handling as zero-cost DWARF exception handling is used by default
  in X86.

llvm-svn: 165989