[LoongArch][Codegen] Add support for TLSDESC

The implementation only enables when the `-enable-tlsdesc` option is
passed and the TLS model is `dynamic`.

LoongArch's GCC has the same option(-mtls-dialet=) as RISC-V.

Reviewers: heiher, MaskRay, SixWeining

Reviewed By: SixWeining, MaskRay

Pull Request: https://github.com/llvm/llvm-project/pull/90159