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.

src/runtime/baremetal.go

@@ -46,6 +46,11 @@ func libc_free(ptr unsafe.Pointer) {
 	free(ptr)
 }
 
+//export runtime_putchar
+func runtime_putchar(c byte) {
+	putchar(c)
+}
+
 //go:linkname syscall_Exit syscall.Exit
 func syscall_Exit(code int) {
 	exit(code)