Optimize vtable calls (#20696)

Optimize vtable calls (#20696)

* Implementation of R2R vtable call thunks. These thunks will fetch the target code pointer from the vtable of the input thisPtr, and jump to that address.
This is especially helpful with generics, since we can avoid a generic dictionary lookup cost for a simple vtable call.
Overall, these thunks cause the CPU to have less branch mispredictions, and give a small performance boost to vtable calls.

These stubs are under VirtualCallStubManager so that the managed debugger can handle stepping through them.