cmd/go: pass expected version to compiler #19064
Description
Split off from discussion #18678, there referred to as "2".
Plan is to add a new internal-use flag to the compiler, -needversion,
and the compiler dies with a useful error if its own runtime.Version
does not match the passed in flag. When invoking the compiler,
the go command would pass -needversion with its own runtime.Version.
The check would then detect when the go command and the compiler
disagree about the Go toolchain version, which happens (to sometimes
mysterious effect) when the first 'go' in $PATH does not match the
$GOROOT environment setting.