[llvm] [LoongArch] Support R_LARCH_{ADD, SUB}_ULEB128 for .uleb128 and force relocs when sym is not in section (PR #76433)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> Unlike RISCV, LoongArch needs padding zero as its contents.

The located content is zero.

Add a description why it needs to be zero.

https://github.com/llvm/llvm-project/pull/76433