cmd/compile: cleanup gdata slightly

cmd/compile: cleanup gdata slightly

In sinit.go, gdata can already handle strings and complex, so no
reason to handle them separately.

In obj.go, inline gdatastring and gdatacomplex into gdata, since it's
the only caller. Allows extracting out the common Linksym calls.

Passes toolstash -cmp.

Change-Id: I3cb18d9b4206a8a269c36e0d30a345d8e6caba1f
Reviewed-on: https://go-review.googlesource.com/31498
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>