[llvm] [LoongArch] Add relax feature and keep relocations (PR #72191)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Frankly, I am glad that I managed to fix the RISC-V assembler issues before you folks are adding the LoongArch side support... Because otherwise some assembler issue would be more difficult to fix once LoongArch builds dependency on the brittle basis...

https://github.com/llvm/llvm-project/pull/72191