Add loongarch64 support

Adding support for LoongArch64 architecture. Tested on Loongson 3A6000 EVB
Board. Since Loongson's SIMD extended instruction set
([LSX](https://gcc.gnu.org/onlinedocs/gcc/LoongArch-SX-Vector-Intrinsics.html),
[LASX](https://gcc.gnu.org/onlinedocs/gcc/LoongArch-ASX-Vector-Intrinsics.html))
is already supported by GCC, more optimizations are being developed.

Here's the benchmark result for Loongson 3A6000 (4c8t, 2.5Ghz) without SIMD
optimizations.
```
Total time (ms) : 17903
Nodes searched  : 1244386
Nodes/second    : 69507
```

closes https://github.com/official-stockfish/Stockfish/pull/4913

No functional change