Add __attribute_optimization_barrier__ to disable inlining and cloning on a
function.  For Clang, expand it to

__attribute__ ((optnone))

Otherwise, expand it to

__attribute__ ((noinline, clone))

Co-Authored-By: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Sam James <sam@gentoo.org>

When _FORTIFY_SOURCE is defined to 2, ____longjmp_chk is called,
instead of longjmp.  ____longjmp_chk compares the relative stack
values to decide if it is called from a stack frame which called
setjmp.  If not, ____longjmp_chk assumes that an alternate signal
stack is used.  Since comparing the relative stack values isn't
reliable with user context, when there is no signal, ____longjmp_chk
will fail.  Undefine _FORTIFY_SOURCE to avoid ____longjmp_chk in
user context test.

Verify that longjmp works correctly after setcontext is called to switch
to a user context.
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>