wasm: use wasi ABI for basic startup/stdout

This allows TinyGo-built binaries to run under wasmtime, for example:

    tinygo build -o test.wasm -no-debug -target=wasm examples/test
    wasmtime run test.wasm 0

targets/wasm_exec.js

@@ -181,31 +181,37 @@
 
 			const timeOrigin = Date.now() - performance.now();
 			this.importObject = {
-				env: {
-					io_get_stdout: function() {
-						return 1;
-					},
-
-					resource_write: function(fd, ptr, len) {
+				wasi_unstable: {
+					// https://github.com/bytecodealliance/wasmtime/blob/master/docs/WASI-api.md#__wasi_fd_write
+					fd_write: function(fd, iovs_ptr, iovs_len, nwritten_ptr) {
+						let nwritten = 0;
 						if (fd == 1) {
-							for (let i=0; i<len; i++) {
-								let c = mem().getUint8(ptr+i);
-								if (c == 13) { // CR
-									// ignore
-								} else if (c == 10) { // LF
-									// write line
-									let line = decoder.decode(new Uint8Array(logLine));
-									logLine = [];
-									console.log(line);
-								} else {
-									logLine.push(c);
+							for (let iovs_i=0; iovs_i<iovs_len;iovs_i++) {
+								let iov_ptr = iovs_ptr+iovs_i*8; // assuming wasm32
+								let ptr = mem().getUint32(iov_ptr + 0, true);
+								let len = mem().getUint32(iov_ptr + 4, true);
+								for (let i=0; i<len; i++) {
+									let c = mem().getUint8(ptr+i);
+									if (c == 13) { // CR
+										// ignore
+									} else if (c == 10) { // LF
+										// write line
+										let line = decoder.decode(new Uint8Array(logLine));
+										logLine = [];
+										console.log(line);
+									} else {
+										logLine.push(c);
+									}
 								}
 							}
 						} else {
 							console.error('invalid file descriptor:', fd);
 						}
+						mem().setUint32(nwritten_ptr, nwritten, true);
+						return 0;
 					},
-
+				},
+				env: {
 					// func ticks() float64
 					"runtime.ticks": () => {
 						return timeOrigin + performance.now();
@@ -344,7 +350,7 @@
 						setTimeout(resolve, 0); // make sure it is asynchronous
 					};
 				});
-				this._inst.exports.cwa_main();
+				this._inst.exports._start();
 				if (this.exited) {
 					break;
 				}