[lld] [lld] Add infrastructure for handling RISCV vendor-specific relocations. (PR #159987)

[Fangrui Song via llvm-commits]

MaskRay wrote:

You can eliminate the pre-scan `if (ctx.arg.emachine == EM_S390)` and post-scan code, unrelated to RISC-V. Also SHT_REL handling. Then call `process` instead of `scan`.

Some boilerplate is expected. This is a critical step as we focus more on performance and make relocation scanner more efficient and competitive with mold and wild.

https://github.com/llvm/llvm-project/pull/159987