x86-64: Improve EVEX strcmp with masked load

In strcmp-evex.S, to compare 2 32-byte strings, replace

        VMOVU   (%rdi, %rdx), %YMM0
        VMOVU   (%rsi, %rdx), %YMM1
        /* Each bit in K0 represents a mismatch in YMM0 and YMM1.  */
        VPCMP   $4, %YMM0, %YMM1, %k0
        VPCMP   $0, %YMMZERO, %YMM0, %k1
        VPCMP   $0, %YMMZERO, %YMM1, %k2
        /* Each bit in K1 represents a NULL in YMM0 or YMM1.  */
        kord    %k1, %k2, %k1
        /* Each bit in K1 represents a NULL or a mismatch.  */
        kord    %k0, %k1, %k1
        kmovd   %k1, %ecx
        testl   %ecx, %ecx
        jne     L(last_vector)

with

        VMOVU   (%rdi, %rdx), %YMM0
        VPTESTM %YMM0, %YMM0, %k2
        /* Each bit cleared in K1 represents a mismatch or a null CHAR
           in YMM0 and 32 bytes at (%rsi, %rdx).  */
        VPCMP   $0, (%rsi, %rdx), %YMM0, %k1{%k2}
        kmovd   %k1, %ecx
        incl    %ecx
        jne     L(last_vector)

It makes EVEX strcmp faster than AVX2 strcmp by up to 40% on Tiger Lake
and Ice Lake.

Co-Authored-By: Noah Goldstein <goldstein.w.n@gmail.com>
(cherry picked from commit c46e9afb)