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.txt

@@ -0,0 +1,11 @@
+called init
+hello!
+called add: 3 5
+called add: 7 9
+called add: 6 1
+reentrantCall: 2 3
+called add: 2 3
+reentrantCall result: 5
+reentrantCall: 1 8
+called add: 1 8
+reentrantCall result: 9