[llvm][LoongArch] Add PC-relative address materialization using pcadd instructions (#175358)
This patch adds support for PC-relative address materialization using
pcadd-class relocations, covering the HI20/LO12 pair and their GOT and
TLS variants (IE, LD, GD, and DESC).
Link:
https://gcc.gnu.org/pipermail/gcc-patches/2025-December/703312.html