selftests/mm: use sys_pkey helpers consistently

sys_pkey_alloc, sys_pkey_free and sys_mprotect_pkey are currently used in
protections_keys.c, while pkey_sighandler_tests.c calls the libc wrappers
directly (e.g.  pkey_mprotect()).  This is probably ok when using glibc
(those symbols appeared a while ago), but Musl does not currently provide
them.  The logging in the helpers from pkey-helpers.h can also come in
handy.

Make things more consistent by using the sys_pkey helpers in
pkey_sighandler_tests.c too.  To that end their implementation is moved to
a common .c file (pkey_util.c).  This also enables calling
is_pkeys_supported() outside of protections_keys.c, since it relies on
sys_pkey_{alloc,free}.

[kevin.brodsky@arm.com: fix dependency on pkey_util.c]
  Link: https://lkml.kernel.org/r/20241216092849.2140850-1-kevin.brodsky@arm.com
Link: https://lkml.kernel.org/r/20241209095019.1732120-12-kevin.brodsky@arm.com


Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Keith Lucas <keith.lucas@oracle.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>