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.

testdata/calls.go

@@ -100,7 +100,7 @@ func deferred(msg string, i int) {
 	println(msg, i)
 }
 
-//go:export __exportedDefer
+//export __exportedDefer
 func exportedDefer() {
 	println("...exported defer")
 }