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_wasip1.go

@@ -13,15 +13,6 @@ type timeUnit int64
 //export __wasm_call_ctors
 func __wasm_call_ctors()
 
-//export _start
-func _start() {
-	// These need to be initialized early so that the heap can be initialized.
-	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
-	heapEnd = uintptr(wasm_memory_size(0) * wasmPageSize)
-	run()
-	__stdio_exit()
-}
-
 // Read the command line arguments from WASI.
 // For example, they can be passed to a program with wasmtime like this:
 //
@@ -100,6 +91,10 @@ func ticks() timeUnit {
 	return timeUnit(nano)
 }
 
+func beforeExit() {
+	__stdio_exit()
+}
+
 // Implementations of WASI APIs
 
 //go:wasmimport wasi_snapshot_preview1 args_get