crypto: sha-mb - Add avx2_supported check.

This patch fixes this allyesconfig target build error with older
binutils.

  LD      arch/x86/crypto/built-in.o
ld: arch/x86/crypto/sha-mb/built-in.o: No such file: No such file or directory

Cc: stable@vger.kernel.org # 3.18+
Signed-off-by: Vinson Lee <vlee@twitter.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>