The minimum GCC version has been bumped to 5.1, so we can get rid of all
the compatibility code for anything older than that.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210922182632.633394-1-ardb@kernel.org