gdbstub: Rename 'softmmu' -> 'system'

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>

gdbstub/internals.h

@@ -103,7 +103,7 @@ static inline int tohex(int v)
 }
 
 /*
- * Connection helpers for both softmmu and user backends
+ * Connection helpers for both system and user backends
  */
 
 void gdb_put_strbuf(void);
@@ -229,7 +229,7 @@ void gdb_breakpoint_remove_all(CPUState *cs);
  * @is_write: is it a write operation
  *
  * This function is specialised depending on the mode we are running
- * in. For softmmu guests we can switch the interpretation of the
+ * in. For system guests we can switch the interpretation of the
  * address to a physical address.
  */
 int gdb_target_memory_rw_debug(CPUState *cs, hwaddr addr,