cmd/go: relax version regexp from CL 547998

cmd/go: relax version regexp from CL 547998

In CL 547998 I relaxed cmd/go's parsing of version lines to allow it
to recognize clang versions with vendor prefixes. To prevent false-positives,
I added a check for a version 3-tuple following the word "version".
However, it appears that some releases of GCC use only a 2-tuple instead.

Updates #64423.
Fixes #64619.

Change-Id: I5f1d0881b6295544a46ab958c6ad4c2155cf51fe
Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest,gotip-windows-amd64-longtest
Reviewed-on: https://go-review.googlesource.com/c/go/+/548120
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>