rts/linker: Consolidate initializer/finalizer handling

Ben Gamarirequested to merge
wip/T21847 into master

Here we extend our treatment of initializer/finalizer priorities to include ELF and in so doing refactor things to share the implementation with PEi386. As well, I fix a subtle misconception of the ordering behavior for .ctors.

Fixes #21847 (closed).

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