x86-64: Require BMI2 for AVX2 (raw|w)memchr implementations

The AVX2 memchr, rawmemchr and wmemchr implementations use the 'bzhi'
and 'sarx' instructions, which belongs to the BMI2 CPU feature.

Fixes: acfd088a ("x86: Optimize memchr-avx2.S")
Partially resolves: BZ #29611

Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>