cmd/compile: accept -lang=go1 as -lang=go1.0

Fixes #65528.

Change-Id: I55fef8cf7be4654c7242462d45f12999e0c91c02
Reviewed-on: https://go-review.googlesource.com/c/go/+/562322
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>

src/cmd/compile/internal/types/goversion.go

@@ -34,7 +34,7 @@ func AllowsGoVersion(major, minor int) bool {
 }
 
 // ParseLangFlag verifies that the -lang flag holds a valid value, and
-// exits if not. It initializes data used by langSupported.
+// exits if not. It initializes data used by AllowsGoVersion.
 func ParseLangFlag() {
 	if base.Flag.Lang == "" {
 		return
@@ -59,6 +59,10 @@ func ParseLangFlag() {
 
 // parseLang parses a -lang option into a langVer.
 func parseLang(s string) (lang, error) {
+	if s == "go1" { // cmd/go's new spelling of "go1.0" (#65528)
+		s = "go1.0"
+	}
+
 	matches := goVersionRE.FindStringSubmatch(s)
 	if matches == nil {
 		return lang{}, fmt.Errorf(`should be something like "go1.12"`)

src/cmd/go/testdata/script/build_issue_65528.txt

@@ -0,0 +1,9 @@
+go build
+
+-- go.mod --
+module test
+
+go 1.0
+
+-- p.go --
+package p