[llvm] [RISCV] Support Global Dynamic TLSDESC in the RISC-V backend (PR #66915)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> The current implementation is only enabled when passing the new `-riscv-enable-tlsdesc` flag.

This needs an update. The cl::opt has been removed.

https://github.com/llvm/llvm-project/pull/66915