Add rx-softmmu

Add rx-softmmu

Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Yoshinori Sato <ysato@users.sourceforge.jp>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
[PMD: Squashed patches from Richard Henderson modifying
      qapi/common.json and tests/machine-none-test.c]
Message-Id: <20200224141923.82118-21-ysato@users.sourceforge.jp>
[PMD: Added @since 5.0 tag in SysEmuTarget]
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>