Use __builtin_offsetof (which is treated as a constant expression) with Clang & GCC. This also allows check_member to work with Clang 9. Signed-off-by: Alex James <theracermaster@gmail.com> Change-Id: I8b5cb4110c13ee42114ecf65932d7f1e5636210e Reviewed-on: https://review.coreboot.org/c/coreboot/+/36249 Reviewed-by: Aaron Durbin <adurbin@chromium.org> Tested-by: build bot (Jenkins) <no-reply@coreboot.org>