[lld] [LLD][COFF] Initial support for ARM64EC importlibs. (PR #107164)

[Jacek Caban via llvm-commits]

cjacek wrote:

Depends on #107162.

This PR introduces a basic importlib test, which is a step towards supporting ARM64EC. However, additional ARM64EC-specific features will be required for full compatibility.

In this PR, the import thunk uses the AMD64 machine type. While this is appropriate for demangled symbols, we also need to implement an ARM64EC variant that handles mangled symbols.

Additionally, there's auxiliary IAT handling that requires more symbols (especially `__imp_aux_*` symbols) and a larger number of chunks overall. For more context, I’ve pushed a draft [here](https://github.com/cjacek/llvm-project/commits/arm64ec-import), and I’ve rebased my arm64ec branch on top of it.

https://github.com/llvm/llvm-project/pull/107164