cmd/go: display cached compiler output more often

CL 77110 arranged for caching and redisplaying compiler output
when reusing a compile artifact from the build cache.

It neglected to redisplay compiler and linker output when avoiding
the compile and link steps by reusing the target output binary
as a cached result. It also neglected to redisplay compiler and linker
output when avoiding the compile and link (and test) steps by reusing
cached test output.

This CL brings back the compiler and linker output in those two cases,
provided it can be found in the build cache. If it can't be found in the
build cache, then the go command still reuses the binaries and avoids
the compile/link/test steps. (It's not worth doing all that work again
just to repeat diagnostic output.)

Fixes #23877.

Change-Id: I25bc054d93a88c039bcb8c5683fe4ac5cb1ee544
Reviewed-on: https://go-review.googlesource.com/128903


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>