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.

Insist a bit harder by adding a compiler_barrier to them.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

tests/arch/arm/arm_mpu_pxn/src/main.c

@@ -18,6 +18,7 @@
  */
 __customramfunc bool custom_ram_func(void)
 {
+	compiler_barrier();
 	return true;
 }
 
@@ -28,6 +29,7 @@ __customramfunc bool custom_ram_func(void)
  */
 __ramfunc bool ram_function(void)
 {
+	compiler_barrier();
 	return true;
 }