Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go release branch 1.9

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"

What did you do?

This was discovered when using u-root (u-root.tk). In u-root we build a minimal standalone environment which lets us build Go programs. There are only 5 binaries:
/init, go, compile, asm, link

We found that go build was failing with the error
go tool: no such tool "pack"

Although in this case pack is not used; e.g.
/go/pkg/tool/linux_amd64/pack r $WORK/math.a $WORK/math/_obj/asin_amd64.o (lots of .o's elided) # internal

To see for yourself:

Create this file as hello.go:
package main
import "fmt"
func main() {
fmt.Println("hi")
}

go build -x -a hello.go
it works.

Then
cp /path/to/go/pkg/tool/$GOOS_$GOARCH/pack /some/where/safe

/path/to/go/pkg/tool/$GOOS_$GOARCH/pack
i.e. just zero out the pack command

go build -x -a hello.go
still works

Then
rm /path/to/go/pkg/tool/$GOOS_$GOARCH/pack

Note: it only fails with -x -a, not with -x or -a.

It may be just something about rebuilding it all, and showing it all?

The special circumstances of u-root may explain why nobody has seen this to date.