x86-64: Require BMI2 and LZCNT for AVX2 memrchr implementation

The AVX2 memrchr implementation uses the 'shlxl' instruction, which
belongs to the BMI2 CPU feature and uses the 'lzcnt' instruction, which
belongs to the LZCNT CPU feature.

Fixes: af5306a7 ("x86: Optimize memrchr-avx2.S")
Partially resolves: BZ #29611

Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>
(cherry picked from commit 3c0c78af)