wasm: do not allow undefined symbols

--allow-undefined can be a problem: it allows compiling code that will
fail when loaded. This change makes sure that if some symbols are
undefined, they are reported as an error by the linker.

Previously, people could get away with importing a function that was not
defined, like this:

    func add(int a, int b) int

    func test() {
        println(add(3, 5))
    }

This was always unintended but mostly worked. With this change, it isn't
possible anymore. Now every function needs to be marked with //export
explicitly:

    //export add
    func add(int a, int b) int

    func test() {
        println(add(3, 5))
    }

As before, functions will be placed in the `env` module with the name
set from the `//export` tag. This can be overridden with
`//go:import-module`:

    //go:import-module math
    //export add
    func add(int a, int b) int

    func test() {
        println(add(3, 5))
    }

For the syscall/js package, I needed to give a list of symbols that are
undefined. This list is based on the JavaScript functions defined in
targets/wasm_exec.js.

src/internal/task/task_asyncify_wasm.S

@@ -2,12 +2,16 @@
 
 .functype start_unwind (i32) -> ()
 .import_module start_unwind, asyncify
+.import_name start_unwind, start_unwind
 .functype stop_unwind () -> ()
 .import_module stop_unwind, asyncify
+.import_name stop_unwind, stop_unwind
 .functype start_rewind (i32) -> ()
 .import_module start_rewind, asyncify
+.import_name start_rewind, start_rewind
 .functype stop_rewind () -> ()
 .import_module stop_rewind, asyncify
+.import_name stop_rewind, stop_rewind
 
 .global  tinygo_unwind
 .hidden  tinygo_unwind