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.