[libc++] Vectorize mismatch (#73255)

```
---------------------------------------------------
Benchmark                           old         new
---------------------------------------------------
bm_mismatch<char>/1           0.835 ns      2.37 ns
bm_mismatch<char>/2            1.44 ns      2.60 ns
bm_mismatch<char>/3            2.06 ns      2.83 ns
bm_mismatch<char>/4            2.60 ns      3.29 ns
bm_mismatch<char>/5            3.15 ns      3.77 ns
bm_mismatch<char>/6            3.82 ns      4.17 ns
bm_mismatch<char>/7            4.29 ns      4.52 ns
bm_mismatch<char>/8            4.78 ns      4.86 ns
bm_mismatch<char>/16           9.06 ns      7.54 ns
bm_mismatch<char>/64           31.7 ns      19.1 ns
bm_mismatch<char>/512           249 ns      8.16 ns
bm_mismatch<char>/4096         1956 ns      44.2 ns
bm_mismatch<char>/32768       15498 ns       501 ns
bm_mismatch<char>/262144     123965 ns      4479 ns
bm_mismatch<char>/1048576    495668 ns     21306 ns
bm_mismatch<short>/1    ...