[LLD] Remove global state in lldCommon

Move all variables at file-scope or function-static-scope into a hosting structure (lld::CommonLinkerContext) that lives at lldMain()-scope. Drivers will inherit from this structure and add their own global state, in the same way as for the existing COFFLinkerContext.

See discussion in https://lists.llvm.org/pipermail/llvm-dev/2021-June/151184.html

Differential Revision: https://reviews.llvm.org/D108850