cmd/dist: do not include runtime/internal/sys in cmd staleness checks

Since CL 454836, cmd/dist has built the packages in 'cmd' with
different settings than those in 'std': namely, for' cmd' we disable
the use of cgo, and (since CL 463740) if GO_BUILDER_NAME is non-empty
or the VERSION file indicates a release version we also set
GOFLAGS=-trimpath.

However, since at least CL 73212 the staleness checks performed by
cmd/dist for the “toolchain” targets (a subset of 'cmd') have included
the package "runtime/internal/sys" (which is in 'std', not 'cmd').
At that time, cmd/go did not have a separate build cache, so it would
not have been possible to check staleness for a 'cmd' build differently
from 'std'. However, now that is possible, and most of the time
"runtime/internal/sys" lives *only* in the build cache (and so is
essentially never stale after building anything that imports it).

But there is one more wrinkle: if GODEBUG=installgoroot=all is set,
the packages in 'std' are still installed to GOROOT/pkg, and can once
again become stale. Since the install with the 'std' configuration does
not match the configuration used to build 'cmd', the staleness check
fails for "runtime/internal/sys" under the 'cmd' configuration.

Since we intentionally build the toolchain with a different
"runtime/internal/sys" stored only in the build cache, there is no
longer a point in checking that package for staleness: if it is stale,
then the toolchain itself will be reported as stale anyway.
So we can simply remove the package from that staleness check,
and unbreak bootstrapping with GODEBUG=installgoroot=all.

I tested this manually using the sequence:

	export GODEBUG=installgoroot=all
	export GO_BUILDER_NAME=linux-amd64-bcmills
	./make.bash

It fails the staleness check before this change, and successfully
builds after.

For golang#24904.

Change-Id: I376e93e35129694a093c6675e20905a097a8b64b
Reviewed-on: https://go-review.googlesource.com/c/go/+/465155
Reviewed-by: Russ Cox <rsc@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>

src/cmd/dist/build.go

@@ -1514,14 +1514,14 @@ func cmdbootstrap() {
 	}
 	goInstall(nil, goBootstrap, "std")
 	goInstall(toolenv(), goBootstrap, "cmd")
-	checkNotStale(toolenv(), goBootstrap, append(toolchain, "runtime/internal/sys")...)
+	checkNotStale(toolenv(), goBootstrap, toolchain...)
 	checkNotStale(nil, goBootstrap, "std")
 	checkNotStale(toolenv(), goBootstrap, "cmd")
 	checkNotStale(nil, gorootBinGo, "std")
 	checkNotStale(toolenv(), gorootBinGo, "cmd")
 	if debug {
 		run("", ShowOutput|CheckExit, pathf("%s/compile", tooldir), "-V=full")
-		checkNotStale(toolenv(), goBootstrap, append(toolchain, "runtime/internal/sys")...)
+		checkNotStale(toolenv(), goBootstrap, toolchain...)
 		copyfile(pathf("%s/compile4", tooldir), pathf("%s/compile", tooldir), writeExec)
 	}