cmd/compile: refactor dtypesym to return *obj.LSym

cmd/compile: refactor dtypesym to return *obj.LSym

All of the callers want a *obj.LSym instead of a *types.Sym, and the
runtime type descriptors don't need Go source symbols anyway.

Passes toolstash-check.

Change-Id: I8ae4b64380fbb547857f49b42465118f41884eed
Reviewed-on: https://go-review.googlesource.com/68251
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>
Reviewed-by: David Crawshaw <crawshaw@golang.org>