all: change //go:export to //export

This is the kind that is used in Go (actually CGo) for exporting
functions. I think it's best to use //export instead of our custom
//go:export pragma, for consistency (they are equivalent in TinyGo).
Therefore I've updated all instances to the standard format (except for
two that are updated in https://github.com/tinygo-org/tinygo/pull/1024).

No smoke tests changed (when comparing the output hash), except for some
wasm tests that include DWARF debug info and tend to be flaky anyway.

src/examples/wasm/export/wasm.go

@@ -8,12 +8,12 @@ import (
 func main() {
 }
 
-//go:export add
+//export add
 func add(a, b int) int {
 	return a + b
 }
 
-//go:export update
+//export update
 func update() {
 	document := js.Global().Get("document")
 	aStr := document.Call("getElementById", "a").Get("value").String()