[lld] [llvm] [ELF] Implement R_RISCV_TLSDESC for RISC-V (PR #79099)

[Fangrui Song via llvm-commits]

MaskRay wrote:

This needs a rebase after https://github.com/llvm/llvm-project/pull/66915 lands. 

I have verified that a runtime test on musl works using this patch
https://gist.github.com/MaskRay/7ad19393fdb0834540db6e7b6b02fa56

I used the same test to find issues for my x86-32 TLSDESC patch https://reviews.llvm.org/D112582

https://github.com/llvm/llvm-project/pull/79099