wasm: use WASI ABI for exit function

This improves compatibility between the regular browser target
(-target=wasm) and the WASI target (-target=wasi). Specifically, it
allows running WASI tests like this:

    tinygo test -target=wasi encoding/base32

src/runtime/runtime_wasm.go

@@ -16,6 +16,12 @@ type __wasi_iovec_t struct {
 //export fd_write
 func fd_write(id uint32, iovs *__wasi_iovec_t, iovs_len uint, nwritten *uint) (errno uint)
 
+// See:
+// https://github.com/WebAssembly/WASI/blob/main/phases/snapshot/docs.md#-proc_exitrval-exitcode
+//go:wasm-module wasi_snapshot_preview1
+//export proc_exit
+func proc_exit(exitcode uint32)
+
 func postinit() {}
 
 const (
@@ -49,6 +55,11 @@ func abort() {
 	trap()
 }
 
+//go:linkname syscall_Exit syscall.Exit
+func syscall_Exit(code int) {
+	proc_exit(uint32(code))
+}
+
 // TinyGo does not yet support any form of parallelism on WebAssembly, so these
 // can be left empty.
 

targets/wasm_exec.js

@@ -276,6 +276,15 @@
 						mem().setUint32(nwritten_ptr, nwritten, true);
 						return 0;
 					},
+					"proc_exit": (code) => {
+						if (global.process) {
+							// Node.js
+							process.exit(code);
+						} else {
+							// Can't exit in a browser.
+							throw 'trying to exit with code ' + code;
+						}
+					},
 				},
 				env: {
 					// func ticks() float64
@@ -289,17 +298,6 @@
 						setTimeout(this._inst.exports.go_scheduler, timeout);
 					},
 
-					// func Exit(code int)
-					"syscall.Exit": (code) => {
-						if (global.process) {
-							// Node.js
-							process.exit(code);
-						} else {
-							// Can't exit in a browser.
-							throw 'trying to exit with code ' + code;
-						}
-					},
-
 					// func finalizeRef(v ref)
 					"syscall/js.finalizeRef": (sp) => {
 						// Note: TinyGo does not support finalizers so this should never be

testdata/stdlib.go

@@ -19,4 +19,7 @@ func main() {
 	// package strings
 	fmt.Println("strings.IndexByte:", strings.IndexByte("asdf", 'd'))
 	fmt.Println("strings.Replace:", strings.Replace("An example string", " ", "-", -1))
+
+	// Exit the program normally.
+	os.Exit(0)
 }