selftests/x86/mm: Add new test that userspace stack is in fact NX

Here is how it works:

* fault and fill the stack from RSP with INT3 down until rlimit allows,

* fill upwards with INT3 too, overwrite libc stuff, argv, envp,

* try to exec INT3 on each page and catch it in either SIGSEGV or
SIGTRAP handler.

Note: trying to execute _every_ INT3 on a 8 MiB stack takes 30-40 seconds
even on fast machine which is too much for kernel selftesting
(not for LTP!) so only 1 INT3 per page is tried.

Tested on F37 kernel and on a custom kernel which does:

vm_flags |= VM_EXEC;

to stack VMA.

Report from the buggy kernel:

$ ./nx_stack_32
stack min ff007000
stack max ff807000
FAIL executable page on the stack: eip ff806001

$ ./nx_stack_64
stack min 7ffe65bb0000
stack max 7ffe663b0000
FAIL executable page on the stack: rip 7ffe663af001

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/4cef8266-ad6d-48af-a5f1-fc2b6a8eb422@p183