x86-64: Require BMI2 for __strlen_evex and __strnlen_evex

Since __strlen_evex and __strnlen_evex added by

commit 1fd8c163
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Fri Mar 5 06:24:52 2021 -0800

    x86-64: Add ifunc-avx2.h functions with 256-bit EVEX

use sarx:

c4 e2 6a f7 c0       	sarx   %edx,%eax,%eax

require BMI2 for __strlen_evex and __strnlen_evex in ifunc-impl-list.c.
ifunc-avx2.h already requires BMI2 for EVEX implementation.

(cherry picked from commit 55bf411b)