cmd/compile/internal/ssa: use obj.LSym instead of gc.Sym

Gc's Sym type represents a package-qualified identifier, which is a
frontend concept and doesn't belong in SSA. Bonus: we can replace some
interface{} types with *obj.LSym.

Passes toolstash -cmp.

Change-Id: I456eb9957207d80f99f6eb9b8eab4a1f3263e9ed
Reviewed-on: https://go-review.googlesource.com/36415


Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>