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/45778
Reviewed-by: Chris Broadfoot <[email protected]>