all: add support for GOARM

This environment variable can be set to 5, 6, or 7 and controls which
ARM version (ARMv5, ARMv6, ARMv7) is used when compiling for GOARCH=arm.

I have picked the default value ARMv6, which I believe is supported on
most common single board computers including all Raspberry Pis. The
difference in code size is pretty big.

We could even go further and support ARMv4 if anybody is interested. It
should be pretty simple to add this if needed.

goenv/goenv.go

@@ -25,6 +25,12 @@ var Keys = []string{
 	"TINYGOROOT",
 }
 
+func init() {
+	if Get("GOARCH") == "arm" {
+		Keys = append(Keys, "GOARM")
+	}
+}
+
 // TINYGOROOT is the path to the final location for checking tinygo files. If
 // unset (by a -X ldflag), then sourceDir() will fallback to the original build
 // directory.
@@ -44,6 +50,20 @@ func Get(name string) string {
 			return dir
 		}
 		return runtime.GOARCH
+	case "GOARM":
+		if goarm := os.Getenv("GOARM"); goarm != "" {
+			return goarm
+		}
+		if goos := Get("GOOS"); goos == "windows" || goos == "android" {
+			// Assume Windows and Android are running on modern CPU cores.
+			// This matches upstream Go.
+			return "7"
+		}
+		// Default to ARMv6 on other devices.
+		// The difference between ARMv5 and ARMv6 is big, much bigger than the
+		// difference between ARMv6 and ARMv7. ARMv6 binaries are much smaller,
+		// especially when floating point instructions are involved.
+		return "6"
 	case "GOROOT":
 		return getGoroot()
 	case "GOPATH":