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 <[email protected]>
(cherry picked from commit 599886a)

cmake/modules/extensions.cmake

@@ -1014,9 +1014,9 @@ endfunction()
 function(zephyr_check_compiler_flag lang option check)
   # Check if the option is covered by any hardcoded check before doing
   # an automated test.
-  zephyr_check_compiler_flag_hardcoded(${lang} "${option}" ${check} exists)
+  zephyr_check_compiler_flag_hardcoded(${lang} "${option}" _${check} exists)
   if(exists)
-    set(${check} ${${check}} PARENT_SCOPE)
+    set(${check} ${_${check}} PARENT_SCOPE)
     return()
   endif()