cmd/go: cache and replay command output during build

cmd/go: cache and replay command output during build

It's nice that

	go build -gcflags=-m errors
	go build -gcflags=-m errors

uses the cache for the second command.

Even nicer is to make the second command
print the same output as the first command.

Fixes #22587.

Change-Id: I64350839f01c86c9a095d9d22f6924cd7a0b9105
Reviewed-on: https://go-review.googlesource.com/77110
Reviewed-by: Ian Lance Taylor <iant@golang.org>