Enable GCToOSInterface to be defined on the GC side of the GC interface (dotnet/corec...

Enable GCToOSInterface to be defined on the GC side of the GC interface (dotnet/coreclr#8121)

* Re-introduce changes lost in a merge conflict

* Enable GCToOSInterface to be defined behind the GC interface
when building the GC in standalone mode. Provide a skeleton Windows
implementation and the framework for a Unix implementation.

* Address code review feedback

Commit migrated from https://github.com/dotnet/coreclr/commit/bb9f30e4658342f83bb3b5639ae24ec92e0a1d2a