cgo: add support for stdio in picolibc and wasi-libc

This adds support for stdio in picolibc and fixes wasm_exec.js so that
it can also support C puts. With this, C stdout works on all supported
platforms.

targets/wasm_exec.js

@@ -256,6 +256,7 @@
 								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);
+								nwritten += len;
 								for (let i=0; i<len; i++) {
 									let c = mem().getUint8(ptr+i);
 									if (c == 13) { // CR
@@ -276,6 +277,9 @@
 						mem().setUint32(nwritten_ptr, nwritten, true);
 						return 0;
 					},
+					fd_close: () => 0,      // dummy
+					fd_fdstat_get: () => 0, // dummy
+					fd_seek: () => 0,       // dummy
 					"proc_exit": (code) => {
 						if (global.process) {
 							// Node.js