Use the common API for semihosting logging.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200603123754.19059-4-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>