[llvm] [RISC-V][LLD] Add Support for RISC-V TLSDESC Relocations (PR #66916)

[Fangrui Song via llvm-commits]

MaskRay wrote:

(will be out of town for ~10 days)

(I did want to implement the TLSDESC stuff myself so I can probably spend less time on reviewing: 
https://github.com/riscv-non-isa/riscv-elf-psabi-doc/issues/94#issuecomment-495966998)

https://github.com/llvm/llvm-project/pull/66916