x86-64: Require BMI2 for strchr-avx2.S [BZ #29611]

Since strchr-avx2.S updated by

commit 1f745ecc
Author: noah <goldstein.w.n@gmail.com>
Date:   Wed Feb 3 00:38:59 2021 -0500

    x86-64: Refactor and improve performance of strchr-avx2.S

uses sarx:

c4 e2 72 f7 c0       	sarx   %ecx,%eax,%eax

for strchr-avx2 family functions, require BMI2 in ifunc-impl-list.c and
ifunc-avx2.h.

This fixes BZ #29611.

(cherry picked from commit 83c5b368)