[dev.link] cmd/compile: make read-only static temps content-addressable

[dev.link] cmd/compile: make read-only static temps content-addressable

For now, we only do this for symbols without relocations.

Mark static temps "local", as they are not referenced across DSO
boundaries. And deduplicating a local symbol and a non-local
symbol can be problematic.

Change-Id: I0a3dc4138aaeea7fd4f326998f32ab6305da8e4b
Reviewed-on: https://go-review.googlesource.com/c/go/+/243141
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jeremy Faller <jeremy@golang.org>