selftests/nolibc: add cc-option compatible with clang cross builds

selftests/nolibc: add cc-option compatible with clang cross builds

The cc-option macro from Build.include is not compatible with clang
cross builds, as it does not respect the "--target" and similar flags,
set up by Mekfile.include.

Provide a custom variant which works correctly.

Reviewed-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20240807-nolibc-llvm-v2-11-c20f2f5fc7c2@weissschuh.net
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>