x86-64: Require BMI2 for AVX2 str(n)casecmp implementations

The AVX2 str(n)casecmp implementations use the 'bzhi' instruction, which
belongs to the BMI2 CPU feature.

NB: It also uses the 'tzcnt' BMI1 instruction, but it is executed as BSF
as BSF if the CPU doesn't support TZCNT, and produces the same result
for non-zero input.

Partially fixes: b77b06e0 ("x86: Optimize strcmp-avx2.S")
Partially resolves: BZ #29611

Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
(cherry picked from commit 10f79d36)