testsuite: Fix up c-c++-common/asan/asan-stack-small.c test

testsuite: Fix up c-c++-common/asan/asan-stack-small.c test

Here is a fix for the test I've talked about today in the libsanitizer
update mail.

The test relied on a coming before b coming before c, all with 32 byte
distances, but gcc can actually emit them in the exact opposite order
or some other one.

2025-11-15  Jakub Jelinek  <jakub@redhat.com>

	* c-c++-common/asan/asan-stack-small.c (pa, pb, pc): Make these
	vars volatile.
	(uintptr_t): New typedef.
	(main): Use access of b using pa pointer with offset depending on
	how exactly the 3 variables are laid out in the frame.