When possible, emit static data rather than
init functions for interface values.

This:

* cuts 32k off cmd/go
* removes several error values from runtime init
* cuts the size of the image/color/palette compiled package from 103k to 34k
* reduces the time to build the package in #15520 from 8s to 1.5s

Fixes #6289
Fixes #15528

Change-Id: I317112da17aadb180c958ea328ab380f83e640b4
Reviewed-on: https://go-review.googlesource.com/26668
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>