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.

testdata/cgo/main.go

@@ -1,6 +1,7 @@
 package main
 
 /*
+#include <stdio.h>
 int fortytwo(void);
 #include "main.h"
 int mul(int, int);
@@ -139,6 +140,10 @@ func main() {
 	buf2 := make([]byte, len(buf1))
 	C.strcpy((*C.char)(unsafe.Pointer(&buf2[0])), (*C.char)(unsafe.Pointer(&buf1[0])))
 	println("copied string:", string(buf2[:C.strlen((*C.char)(unsafe.Pointer(&buf2[0])))]))
+
+	// libc: test basic stdio functionality
+	putsBuf := []byte("line written using C puts\x00")
+	C.puts((*C.char)(unsafe.Pointer(&putsBuf[0])))
 }
 
 func printUnion(union C.joined_t) C.joined_t {

testdata/cgo/out.txt

@@ -62,3 +62,4 @@ option 3A: 21
 enum width matches: true
 CFLAGS value: 17
 copied string: foobar
+line written using C puts