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.

lib/picolibc-stdio.c

@@ -0,0 +1,18 @@
+// This file is included in the picolibc build.
+// It makes stdio functions available to the C library.
+
+#include <stdio.h>
+#include <sys/cdefs.h>
+
+// Defined in the runtime package. Writes to the default console (usually, the
+// first UART or an USB-CDC device).
+int runtime_putchar(char, FILE*);
+
+// Define stdin, stdout, and stderr as a single object.
+// This object must not reside in ROM.
+static FILE __stdio = FDEV_SETUP_STREAM(runtime_putchar, NULL, NULL, _FDEV_SETUP_WRITE);
+
+// Define the underlying structs for stdin, stdout, and stderr.
+FILE *const stdin = &__stdio;
+__strong_reference(stdin, stdout);
+__strong_reference(stdin, stderr);