cmake: fix variable de-referencing in zephyr_check_compiler_x functions

Fixes: #53124

Fix de-referencing of check and exists function arguments by correctly
de-referencing the argument references using `${<var>}`.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
(cherry picked from commit 04a27651)