[Local GC] Enable feature: GC_PROFILING

[swgillespie]

Related to https://github.com/dotnet/coreclr/issues/8268. Some of this work has been done already but there is additional work to be done to enable a profiler to walk the heap - in particular, ScanContext refers to VM internals when traversing the heap.

[swgillespie]

Triage:

This is an important feature for 2.1, if only to prevent issues like dotnet/coreclr#14777 and dotnet/coreclr#14747 from occuring (though obviously the ideal shipping configuration is for profiling to work with a standalone GC).

[davmason]

Fixed in dotnet/coreclr#19777