cmd/go/internal/work: factor build.go into multiple files

build.go - commands and misc helpers
action.go - action graph construction
exec.go - action graph execution
gc.go - gc toolchain
gccgo.go - gccgo toolchain

Change-Id: I39b6e2490ac05334c2321e9ad88df694a6efa82f
Reviewed-on: https://go-review.googlesource.com/70671
Run-TryBot: Russ Cox <[email protected]>
Reviewed-by: David Crawshaw <[email protected]>

Author: rsc