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

[Fangrui Song via llvm-commits]

MaskRay wrote:

I am expecting to see something more complete. There is no rush in adding linker relaxation support, and if not careful, can end up needing more efforts to clean up. This is what happened to RISC-V's assembler support, and I finally fixed them after  https://reviews.llvm.org/D153097 https://reviews.llvm.org/D155357 . At the very least, this patch needs more tests, like the tests I improved when improving the two tests.

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