cmd/go: disable tool version check during bootstrap

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 golang#19064
Fixes golang#20674

Change-Id: Ibdd7a92377f4cc77d71ed548f02d48bde6550f67
Reviewed-on: https://go-review.googlesource.com/45778
Reviewed-by: Chris Broadfoot <[email protected]>

[release-branch.go1.8] go1.8.3

Change-Id: I048f21f8ca68758fdd7ac875f7db5e4ed1930f3b
Reviewed-on: https://go-review.googlesource.com/44037
Reviewed-by: Brad Fitzpatrick <[email protected]>

[release-branch.go1.8] go1.8.2

Change-Id: Ib04878cbfbb0c09fbd0cc614df314c835e9a6eb0
Reviewed-on: https://go-review.googlesource.com/43991
Reviewed-by: Brad Fitzpatrick <[email protected]>

[release-branch.go1.7] go1.7.6

Change-Id: I6361937bb2684c6b64edafc19d7d175210638063
Reviewed-on: https://go-review.googlesource.com/43992
Reviewed-by: Brad Fitzpatrick <[email protected]>

[release-branch.go1.8] go1.8.1

Change-Id: Ieb4552841bbf488acdbde805958a1e2ae0bd8aa3
Reviewed-on: https://go-review.googlesource.com/39920
Reviewed-by: Brad Fitzpatrick <[email protected]>

[release-branch.go1.8] go1.8

Change-Id: If1e38f02db86449abd4c8a57988d9825b1cf2511
Reviewed-on: https://go-review.googlesource.com/37132
Run-TryBot: Chris Broadfoot <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Russ Cox <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

[release-branch.go1.8] go1.8rc3

Change-Id: Ie306bb5355f56113356fc141f3c1a56872b39f9e
Reviewed-on: https://go-review.googlesource.com/35836
Reviewed-by: Chris Broadfoot <[email protected]>

[release-branch.go1.7] go1.7.5

Change-Id: I09b5f6f3c79ec691f6d2fd28551dc06d79105c42
Reviewed-on: https://go-review.googlesource.com/35834
Run-TryBot: Chris Broadfoot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

[release-branch.go1.8] go1.8rc2

Change-Id: Ifcf2e13b962aa10280df8ca76cb21b37e3533f8f
Reviewed-on: https://go-review.googlesource.com/35475
Run-TryBot: Chris Broadfoot <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>

[release-branch.go1.8] go1.8rc1

Change-Id: I68a99a4d750357dd59eb48f7c05b4dc08c64c92d
Reviewed-on: https://go-review.googlesource.com/35097
Run-TryBot: Chris Broadfoot <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Brad Fitzpatrick <[email protected]>