Added memchr as alternative benchmark on 2025/08/08 post

Added `memchr()` from glibc for comparison. ``` | Input Size (bytes) | memchr (GB/s) | std::find (GB/s) | simdutf::find (GB/s) | naive_find (GB/s) | |--------------------|------------------|------------------|-----------------------|-------------------| | 1024 | 105.96 | 4.91 | 28.97 | 3.65 | | 8192 | 174.75 | 5.04 | 50.29 | 3.78 | | 65536 | 120.42 | 5.06 | 57.73 | 3.79 | | 524288 | 101.33 | 5.06 | 58.30 | 3.79 | | 2097152 | 84.29 | 5.06 | 58.19 | 3.80 | ```