musl: add mips optimized memcpy and memset

[SiarheiVolkau]

The musl library has no mips specific optimization for memcpy/memset which are widely used.

The patch aims to port the code for these functions from uClibc-ng.

Performance shall be definitely better but might be not noticeable in some apps.
So If someone can perform the testing that would be great.

Thanks.

[glebm]

Sadly, DevilutionX comparison @ 420 MHz showed no difference.
The asm version might even be like 0.005 FPS slower (though it's hard to tell as it fluctuates more than that).
(These are the OPKs that I used to test it https://github.com/od-contrib/buildroot/releases/tag/od-rs90-max-mem-2022-12-28)