[lld] [LLD][COFF] Add basic ARM64X dynamic relocations support (PR #118035)

[Jacek Caban via llvm-commits]

cjacek wrote:

This commit introduces an initial implementation of ARM64X relocations. While this is a foundational step, further ARM64X support will require additional relocation types. The relocations included here represent the minimal set found in all ARM64X binaries.

The implementation omits untestable elements for now, such as additional relocation types and relocations to non-header pages (e.g., those relative to symbols or chunks). These can be easily added to `Arm64XDynamicRelocEntry` later, but testing them will require a few more basic ARM64X features to be in place.

The ability to create these relocations will be useful elsewhere in the codebase, particularly when constructing import tables. As such, the relocations are stored in `COFFLinkerContext` and created early in the writer phase.

https://github.com/llvm/llvm-project/pull/118035