loongarch: add loongarch simd opt

[HecaiYuan]

The LoongArch created by Loongson is a new RISC ISA, which is a bit like MIPS or RISC-V.

Two extensions providing SIMD instructions have been proposed:

• LSX (Loongson SIMD eXtension) with 128-bit vectors is very similar to SSE on x86;
• LASX (Loongson Advanced SIMD eXtension) with 256-bit vectors is very similar to AVX on x86.

To compile, we run either:
cmake -DKISSFFT_DATATYPE=simd -DHAVE_LSX=ON ../ or make DKISSFFT_DATATYPE=simd HAVE_LSX=lsx.

Further reading

• https://jia.je/unofficial-loongarch-intrinsics-guide/
• https://github.com/loongson/LoongArch-Documentation/releases/latest/download/LoongArch-Vol1-v1.10-EN.pdf
• http://www.loongson.cn/

We are ready for review, please let me know if there are more changes or enhancements that should be made. Thanks for everyone in advance. @mborgerding @basilgello

[HecaiYuan]

Hello @mborgerding @basilgello . Whenever you have some time, could you please provide your insights on this? Your feedback would be very valuable. Thank you.

[HecaiYuan]

Hi @mborgerding @basilgello @C0rn3j , I wanted to check if there's any feedback on this PR. Would be happy to provide more details if helpful.