compiler-gcc.h: __nostackprotector needs gcc-4.4 and up

Gcc versions before 4.4 do not recognize the __optimize__ compiler
attribute:

    warning: ‘__optimize__’ attribute directive ignored

Fixes: 7375ae3a ("compiler-gcc.h: Introduce __nostackprotector function attribute")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>