cmd/go: expand acceptable toolchains a bit

cmd/go: expand acceptable toolchains a bit

Allow both prefix-goVERSION and goVERSION-suffix for custom toolchains.
Also make sure that a tie in the toolchain and min version goes to the
toolchain line.

For #57001.

Change-Id: Ibeea5f47db7349cfdf36da188bb43e195e196f5b
Reviewed-on: https://go-review.googlesource.com/c/go/+/498076
Auto-Submit: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>