crypto: sha1 - use Kbuild supplied flags for AVX test

Commit ea4d26ae ("raid5: add AVX optimized RAID5 checksumming")
introduced x86/ arch wide defines for AFLAGS and CFLAGS indicating AVX
support in binutils based on the same test we have in x86/crypto/ right
now. To minimize duplication drop our implementation in favour to the
one in x86/.

Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>