cmd/compile: move a lot of declarations outside of go.go

cmd/compile: move a lot of declarations outside of go.go

go.go is currently a grab bag of various unrelated type and variable
declarations. Move a bunch of them into other more relevant source
files.

There are still more that can be moved, but these were the low hanging
fruit with obvious homes.

No code/comment changes. Just shuffling stuff around.

Change-Id: I43dbe1a5b8b707709c1a3a034c693d38b8465063
Reviewed-on: https://go-review.googlesource.com/21561
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>