compiler-gcc.h: Introduce __optimize function attribute

Create a new function attribute __optimize, which allows to specify an
optimization level on a per-function basis.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>