sandbox: replace putchar(ch) by fputc(ch, stdout)

sandbox: replace putchar(ch) by fputc(ch, stdout)

When compiled with -Og for better debugability u-boot ends up in a stack
overflow using

    gcc (Ubuntu 11.2.0-7ubuntu2) 11.2.0
    GNU Binutils for Ubuntu 2.37

putchar(ch) is defined as a macro which ends up calling U-Boot's putc()
implementation instead of the glibc one, which calls os_putc() ...

Let's use fputc(ch, stdout) instead as fputc() does not exist in U-Boot.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>