Don't pull in setjmp.h in -ffreestanding compiles.

Also provide _setjmpex(). r200243 put in _setjmp() and _setjmpex() behind a
comment since jmp_buf wasn't available. r200344 added jmp_buf and put in
_setjmp(), but missed _setjmpex().


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212557 91177308-0d34-0410-b5e6-96231b3b80d8