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.

testdata/wasmexport-noscheduler.go

@@ -0,0 +1,35 @@
+package main
+
+func init() {
+	println("called init")
+}
+
+//go:wasmimport tester callTestMain
+func callTestMain()
+
+func main() {
+	// main.main is not used when using -buildmode=c-shared.
+	callTestMain()
+}
+
+//go:wasmexport hello
+func hello() {
+	println("hello!")
+}
+
+//go:wasmexport add
+func add(a, b int) int {
+	println("called add:", a, b)
+	return a + b
+}
+
+//go:wasmimport tester callOutside
+func callOutside(a, b int) int
+
+//go:wasmexport reentrantCall
+func reentrantCall(a, b int) int {
+	println("reentrantCall:", a, b)
+	result := callOutside(a, b)
+	println("reentrantCall result:", result)
+	return result
+}