[dev.link] cmd/internal/obj: use index for static symbols

[dev.link] cmd/internal/obj: use index for static symbols

In assembly we always reference symbols by name. But for static
symbols, as they are reachable only within the current file, we
can assign them local indices and use the indices to reference
them. The index is only meaningful locally, and it is fine.

Change-Id: I16e011cd41575ef703ceb6f35899e5fa58fbcf1e
Reviewed-on: https://go-review.googlesource.com/c/go/+/201997
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>