semihosting: Improve condition for config.c and console.c

While CONFIG_SEMIHOSTING is currently only set for softmmu,
this will not continue to be true.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

semihosting/meson.build

-specific_ss.add(when: 'CONFIG_SEMIHOSTING', if_true: files(
+specific_ss.add(when: ['CONFIG_SEMIHOSTING', 'CONFIG_SOFTMMU'], if_true: files(
   'config.c',
   'console.c',
 ))