cmake: compiler: gcc: Explicitly spell out stddef.h and limits.h

Depending on how compiler is built, it prints a different include
directory with `--print-file-name=include`.  What we want, instead, is
directories with `stddef.h` and `include-fixed/limits.h`.

This commit explicitly specify the header files we want to use, then
take the directory from the returned path.

Signed-off-by: Yasushi SHOJI <y-shoji@ispace-inc.com>