Add SIMD impl of memchr2 for LoongArch

[heiher]

Benchmark results on LA664:

• LASX

simd/memchr2/8          time:   [7.6037 ns 7.6060 ns 7.6094 ns]
                        thrpt:  [1.1015 GiB/s 1.1020 GiB/s 1.1023 GiB/s]
                 change:
                        time:   [−5.0872% −5.0162% −4.9421%] (p = 0.00 < 0.05)
                        thrpt:  [+5.1990% +5.2812% +5.3599%]
                        Performance has improved.

simd/memchr2/136        time:   [10.450 ns 10.460 ns 10.470 ns]
                        thrpt:  [12.186 GiB/s 12.198 GiB/s 12.209 GiB/s]
                 change:
                        time:   [−90.586% −90.570% −90.555%] (p = 0.00 < 0.05)
                        thrpt:  [+958.73% +960.49% +962.21%]
                        Performance has improved.

simd/memchr2/1024       time:   [44.428 ns 44.456 ns 44.492 ns]
                        thrpt:  [21.456 GiB/s 21.473 GiB/s 21.487 GiB/s]
                 change:
                        time:   [−94.643% −94.633% −94.625%] (p = 0.00 < 0.05)
                        thrpt:  [+1760.5% +1763.2% +1766.7%]
                        Performance has improved.

simd/memchr2/131072     time:   [4.9362 µs 4.9389 µs 4.9425 µs]
                        thrpt:  [24.698 GiB/s 24.716 GiB/s 24.730 GiB/s]
                 change:
                        time:   [−95.301% −95.296% −95.292%] (p = 0.00 < 0.05)
                        thrpt:  [+2024.1% +2025.9% +2027.9%]
                        Performance has improved.

simd/memchr2/134217728  time:   [5.7547 ms 5.7604 ms 5.7662 ms]
                        thrpt:  [21.678 GiB/s 21.700 GiB/s 21.721 GiB/s]
                 change:
                        time:   [−94.648% −94.642% −94.637%] (p = 0.00 < 0.05)
                        thrpt:  [+1764.7% +1766.5% +1768.5%]
                        Performance has improved.

• LSX

simd/memchr2/8          time:   [3.6022 ns 3.6041 ns 3.6066 ns]
                        thrpt:  [2.3241 GiB/s 2.3257 GiB/s 2.3269 GiB/s]
                 change:
                        time:   [−55.037% −55.002% −54.966%] (p = 0.00 < 0.05)
                        thrpt:  [+122.05% +122.23% +122.41%]
                        Performance has improved.

simd/memchr2/136        time:   [10.407 ns 10.414 ns 10.422 ns]
                        thrpt:  [12.243 GiB/s 12.252 GiB/s 12.260 GiB/s]
                 change:
                        time:   [−90.624% −90.609% −90.592%] (p = 0.00 < 0.05)
                        thrpt:  [+962.91% +964.87% +966.60%]
                        Performance has improved.

simd/memchr2/1024       time:   [55.633 ns 55.662 ns 55.702 ns]
                        thrpt:  [17.138 GiB/s 17.150 GiB/s 17.159 GiB/s]
                 change:
                        time:   [−93.292% −93.281% −93.272%] (p = 0.00 < 0.05)
                        thrpt:  [+1386.4% +1388.2% +1390.9%]
                        Performance has improved.

simd/memchr2/131072     time:   [6.5801 µs 6.5845 µs 6.5899 µs]
                        thrpt:  [18.524 GiB/s 18.539 GiB/s 18.551 GiB/s]
                 change:
                        time:   [−93.736% −93.729% −93.723%] (p = 0.00 < 0.05)
                        thrpt:  [+1493.2% +1494.7% +1496.4%]
                        Performance has improved.

simd/memchr2/134217728  time:   [7.1782 ms 7.1828 ms 7.1887 ms]
                        thrpt:  [17.388 GiB/s 17.403 GiB/s 17.414 GiB/s]
                 change:
                        time:   [−93.324% −93.319% −93.314%] (p = 0.00 < 0.05)
                        thrpt:  [+1395.6% +1396.9% +1398.0%]
                        Performance has improved.