meson: Rename target_softmmu_arch -> target_system_arch

meson: Rename target_softmmu_arch -> target_system_arch

Finish the convertion started with commit de6cd7599b
("meson: Replace softmmu_ss -> system_ss"). If the
$target_type is 'system', then use the target_system_arch[]
source set :)

Mechanical change doing:

  $ sed -i -e s/target_softmmu_arch/target_system_arch/g \
      $(git grep -l target_softmmu_arch)

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231004090629.37473-13-philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>