[llvm] [llvm-mc] Add --load to support dynamically loaded plugins (PR #222983)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke commented:

Hm, we're currently moving in the other direction of removing PluginLoader.h (see e.g. https://github.com/llvm/llvm-project/pull/217860)..

https://github.com/llvm/llvm-project/pull/222983