Initial implementation of generic dictionary access for R2R. (dotnet/coreclr#4519)

Initial implementation of generic dictionary access for R2R. (dotnet/coreclr#4519)

For now, only TypeHandle dictionary entry slots are supported and encoded in a R2R version resilient format (the rest to come soon).
Support is only limited for x64 Windows platforms (rest is still TODO)

The basic idea: each dictionary access is initally a call to a DynamicHelper R2R cell that computes the dictionary signature, and patches the R2R cell address with an assembly stub that performs the dictionary lookup.

Commit migrated from https://github.com/dotnet/coreclr/commit/97b4ff0b438261ba11b357008630076054a6f25d