wasm: add //go:wasmexport support (#4451)

This adds support for the `//go:wasmexport` pragma as proposed here:
https://github.com/golang/go/issues/65199

It is currently implemented only for wasip1 and wasm-unknown, but it is
certainly possible to extend it to other targets like GOOS=js and
wasip2.

main.go

@@ -1500,6 +1500,7 @@ func main() {
 	var tags buildutil.TagsFlag
 	flag.Var(&tags, "tags", "a space-separated list of extra build tags")
 	target := flag.String("target", "", "chip/board name or JSON target specification file")
+	buildMode := flag.String("buildmode", "", "build mode to use (default, c-shared)")
 	var stackSize uint64
 	flag.Func("stack-size", "goroutine stack size (if unknown at compile time)", func(s string) error {
 		size, err := bytesize.Parse(s)
@@ -1608,6 +1609,7 @@ func main() {
 		GOARM:           goenv.Get("GOARM"),
 		GOMIPS:          goenv.Get("GOMIPS"),
 		Target:          *target,
+		BuildMode:       *buildMode,
 		StackSize:       stackSize,
 		Opt:             *opt,
 		GC:              *gc,