[lld][ELF] Support LoongArch

This adds support for the LoongArch ELF psABI v2.00 [1] relocation
model to LLD. The deprecated stack-machine-based psABI v1 relocs are not
supported.

The code is tested by successfully bootstrapping a Gentoo/LoongArch
stage3, complete with common GNU userland tools and both the LLVM and
GNU toolchains (GNU toolchain is present only for building glibc,
LLVM+Clang+LLD are used for the rest). Large programs like QEMU are
tested to work as well.

[1]: https://loongson.github.io/LoongArch-Documentation/LoongArch-ELF-ABI-EN.html

Reviewed By: MaskRay, SixWeining

Differential Revision: https://reviews.llvm.org/D138135