[llvm] [LoongArch] Add R_LARCH_MARK_LA relocation for la.abs (PR #161062)

[via llvm-commits]

heiher wrote:

In the [la-abi-specs](https://github.com/loongson/la-abi-specs/blob/release/laelf.adoc), `R_LARCH_MARK_LA` is described as being used to mark `la.abs`. Should this be backported to LLVM 21?

https://github.com/llvm/llvm-project/pull/161062