Avoid unnecessary return val copying in tailcall-via-help (#72720)

Avoid unnecessary return val copying in tailcall-via-help (#72720)

The initial implementation of this did not handle the fact that retbuf
can point to GC heap during reflection invoke. It was fixed in #39815,
but the way it was fixed was by copying it into a local. This changes
the fix so that we simply report the return value pointer as a byref
throughout the mechanism, which simplifies the JIT's handling and is a
perf improvement as well.