[Local GC] Scaffolding for loading a standalone GC (dotnet/coreclr#11242)

[Local GC] Scaffolding for loading a standalone GC (dotnet/coreclr#11242)

* Configure the build system to build a CoreCLR capable of loading a standalone GC

* Proto-implementation of dynamic GC loading

* Build the GC with the VM's CMakeLists when doing a non-standalone build of the GC

* [Local GC] Introduce a new feature define, FEATURE_STANDALONE_GC_ONLY, to be used by the CI to explicitly test local GC dynamic loading code paths

* Fix the FEATURE_STANDALONE_GC_ONLY build for unix linkers

* Rebase against master

* Code review feedback: use the existing Unix exports file

Commit migrated from https://github.com/dotnet/coreclr/commit/1a183684b1ecf63ece8a2fd80173f083c0deea52