selftests/mm: build with -O2

The mm kselftests are currently built with no optimisation (-O0).  It's
unclear why, and besides being obviously suboptimal, this also prevents
the pkeys tests from working as intended.  Let's build all the tests with
-O2.

[kevin.brodsky@arm.com: silence unused-result warnings]
  Link: https://lkml.kernel.org/r/20250107170110.2819685-1-kevin.brodsky@arm.com
Link: https://lkml.kernel.org/r/20241209095019.1732120-6-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>