compiler_attributes.h: cleanups for GCC 4.9+

Since
commit 6ec4476a ("Raise gcc version requirement to 4.9")
we no longer support building the kernel with GCC 4.8; drop the
preprocess checks for __GNUC_MINOR__ version. It's implied that if
__GNUC_MAJOR__ is 4, then the only supported version of __GNUC_MINOR__
left is 9.

Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210621231822.2848305-3-ndesaulniers@google.com