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.

src/runtime/runtime_tinygowasm.go

@@ -1,5 +1,8 @@
 //go:build tinygo.wasm && !wasm_unknown && !wasip2
 
+// This file is for wasm/wasip1 and for wasm/js, which both use much of the
+// WASIp1 API.
+
 package runtime
 
 import (