compiler-gcc.h: drop checks for older GCC versions

Now that GCC 5.1 is the minimally supported default, drop the values we
don't use.

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>