cmake: prefix local version of return variable

Fixes: #55490
Follow-up: #53124

Prefix local version of the return variable before calling
`zephyr_check_compiler_flag_hardcoded()`.

This ensures that there will never be any naming collision between named
return argument and the variable name used in later functions when
PARENT_SCOPE is used.

The issue #55490 provided description of situation where the double
de-referencing was not working correctly.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>