We have gdbstub/user.c for user emulation code,
use gdbstub/system.c for system emulation part.

Rename s/softmmu/system/ in meson and few comments.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20231004090629.37473-8-philmd@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>