tests/arm_mpu_pxn: Insist that trivial functions are not inlined

GCC 14 takes a look these functions which are just 'return true' and
inlines them even though they has the noinline attribute set. This happens
because the compiler computes the possible range of values from the
function call, and as that is a single value (true), it replaces the call
with that value. So it's not strictly inlining the function?

Insist a bit harder by adding a compiler_barrier to the functions which
seems to solve the issue.

Signed-off-by: Keith Packard <keithp@keithp.com>