cmd/go: add GOWASM environment variable

This change adds the environment variable GOWASM, which is a comma
separated list of experimental WebAssembly features that the compiled
WebAssembly binary is allowed to use. The default is to use no
experimental features. Initially there are no features avaiable.

More information about feature proposals can be found at
https://github.com/WebAssembly/proposals

Change-Id: I4c8dc534c99ecff8bb075dded0186ca8f8decaef
Reviewed-on: https://go-review.googlesource.com/c/go/+/168881
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/objabi/util.go

@@ -29,6 +29,7 @@ var (
 	GOMIPS   = gomips()
 	GOMIPS64 = gomips64()
 	GOPPC64  = goppc64()
+	GOWASM   = gowasm()
 	GO_LDSO  = defaultGO_LDSO
 	Version  = version
 )
@@ -76,6 +77,29 @@ func goppc64() int {
 	panic("unreachable")
 }
 
+type gowasmFeatures struct {
+	// no features yet
+}
+
+func (f *gowasmFeatures) String() string {
+	var flags []string
+	// no features yet
+	return strings.Join(flags, ",")
+}
+
+func gowasm() (f gowasmFeatures) {
+	for _, opt := range strings.Split(envOr("GOWASM", ""), ",") {
+		switch opt {
+		// no features yet
+		case "":
+			// ignore
+		default:
+			log.Fatalf("Invalid GOWASM value. No such feature: " + opt)
+		}
+	}
+	return
+}
+
 func Getgoextlinkenabled() string {
 	return envOr("GO_EXTLINK_ENABLED", defaultGO_EXTLINK_ENABLED)
 }