wasm: use wasip1 API for parameter/env passing

Instead of hardcoding the command line parameters and the environment
variables in the binary, pass them at runtime to Node.js and use the
WASIp1 API to retrieve them in wasm_exec.js.

The only real benefit right now is that it becomes possible to change
`go.argv` and `go.env` before running a wasm binary.

This also changes the syscall package for GOOS=js: it now becomes more
like a libc (using wasi-libc), which means error values like
`syscall.EEXIST` will actually match the one returned by the relevant
libc function.

Wasm binary size for packages that import the os package will be
increased somewhat.

targets/wasm_exec.js

@@ -135,6 +135,8 @@
 
 	global.Go = class {
 		constructor() {
+			this.argv = ["js"];
+			this.env = {};
 			this._callbackTimeouts = new Map();
 			this._nextCallbackTimeoutID = 1;
 
@@ -235,9 +237,58 @@
 				return decoder.decode(new DataView(this._inst.exports.memory.buffer, ptr, len));
 			}
 
+			const storeStringArraySizes = (array, num_ptr, buf_size_ptr) => {
+				let buf_size = 0;
+				for (let s of array) {
+					buf_size += s.length + 1;
+				}
+				mem().setUint32(num_ptr, array.length, true);
+				mem().setUint32(buf_size_ptr, buf_size, true);
+			}
+
+			const storeStringArray = (array, ptrs_ptr, buf_ptr) => {
+				for (let s of array) {
+					// Put string data in buffer.
+					let data = encoder.encode(s);
+					let dest = new Uint8Array(this._inst.exports.memory.buffer, buf_ptr, data.length);
+					dest.set(data);
+					mem().setUint8(buf_ptr+data.length, 0);
+
+					// Put pointer to buffer in pointers array.
+					mem().setUint32(ptrs_ptr, buf_ptr, true);
+
+					// Advance to the next element in the array.
+					ptrs_ptr += 4;
+					buf_ptr += data.length + 1;
+				}
+			}
+
+			const envArray = () => {
+				let array = [];
+				for (let [key, value] of Object.entries(this.env)) {
+					array.push(`${key}=${value}`);
+				}
+				return array;
+			}
+
 			const timeOrigin = Date.now() - performance.now();
 			this.importObject = {
 				wasi_snapshot_preview1: {
+					args_sizes_get: (argc_ptr, argv_buf_size_ptr) => {
+						storeStringArraySizes(this.argv, argc_ptr, argv_buf_size_ptr);
+						return 0;
+					},
+					args_get: (argv_ptr, argv_buf_ptr) => {
+						storeStringArray(this.argv, argv_ptr, argv_buf_ptr);
+						return 0;
+					},
+					environ_get: (env_ptr, env_buf_ptr) => {
+						storeStringArray(envArray(), env_ptr, env_buf_ptr);
+					},
+					environ_sizes_get: (env_ptr, env_buf_size_ptr) => {
+						storeStringArraySizes(envArray(), env_ptr, env_buf_size_ptr);
+						return 0;
+					},
 					// https://github.com/WebAssembly/WASI/blob/main/phases/snapshot/docs.md#fd_write
 					fd_write: function(fd, iovs_ptr, iovs_len, nwritten_ptr) {
 						let nwritten = 0;
@@ -504,12 +555,14 @@
 		global.process.versions &&
 		!global.process.versions.electron
 	) {
-		if (process.argv.length != 3) {
+		if (process.argv.length < 3) {
 			console.error("usage: go_js_wasm_exec [wasm binary] [arguments]");
 			process.exit(1);
 		}
 
 		const go = new Go();
+		go.argv = process.argv.slice(2);
+		go.env = process.env;
 		WebAssembly.instantiate(fs.readFileSync(process.argv[2]), go.importObject).then((result) => {
 			return go.run(result.instance);
 		}).catch((err) => {