cmd/internal/obj: validate GOARM environment variable's value before use

I was previously setting GOARM=arm5 (due to confusion with previously
seeing buildall.sh's temporary of "arm5" as a GOARCH and
misremembernig), but GOARM=arm5 was acting like GOARM=5 only on
accident. See https://go-review.googlesource.com/#/c/10023/

Instead, fail if GOARM is not a known value.

Change-Id: I9ba4fd7268df233d40b09f0431f37cd85a049847
Reviewed-on: https://go-review.googlesource.com/10024
Reviewed-by: Minux Ma <minux@golang.org>

src/cmd/internal/obj/util.go

@@ -213,10 +213,17 @@ func Getgoos() string {
 }
 
 func Getgoarm() string {
-	return envOr("GOARM", defaultGOARM)
+	switch v := envOr("GOARM", defaultGOARM); v {
+	case "5", "6", "7":
+		return v
+	}
+	// Fail here, rather than validate at multiple call sites.
+	log.Fatalf("Invalid GOARM value. Must be 5, 6, or 7.")
+	panic("unreachable")
 }
 
 func Getgo386() string {
+	// Validated by cmd/8g.
 	return envOr("GO386", defaultGO386)
 }