The check that the go tool version matched the go compiler version was
too aggressive and didn't cover the bootstrapping case with make.bash.

We never noticed because we never had a VERSION file in place.

Repro:

$ echo "go1.9beta1" > $GOROOT/VERSION
$ cd $GOROOT/src
$ ./make.bash

No test, because x/build/cmd/release catches it.

Updates #19064
Fixes #20674

Change-Id: Ibdd7a92377f4cc77d71ed548f02d48bde6550f67
Reviewed-on: https://go-review.googlesource.com/45778Reviewed-by: Chris Broadfoot <cbro@golang.org>