Mark asm goto with outputs as volatile

The manual references asm goto as being implicitly volatile already
and that was done when asm goto could not have outputs. When outputs
were added to `asm goto`, only asm goto without outputs were still being
marked as volatile. Now some parts of GCC decide, removing the `asm goto`
is ok if the output is not used, though not updating the CFG (this happens
on both the RTL level and the gimple level). Since the biggest user of `asm goto`
is the Linux kernel and they expect them to be volatile (they use them to
copy to/from userspace), we should just mark the inline-asm as volatile.

OK? Bootstrapped and tested on x86_64-linux-gnu.

	PR middle-end/110420
	PR middle-end/103979
	PR middle-end/98619

gcc/ChangeLog:

	* gimplify.cc (gimplify_asm_expr): Mark asm with labels as volatile.

gcc/testsuite/ChangeLog:

	* gcc.c-torture/compile/asmgoto-6.c: New test.

(cherry picked from commit 478840a2)