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>
diff --git a/gdbstub/trace-events b/gdbstub/trace-events
index 7bc79a7..4fd126a 100644
--- a/gdbstub/trace-events
+++ b/gdbstub/trace-events
@@ -28,5 +28,5 @@
 gdbstub_err_checksum_incorrect(uint8_t expected, uint8_t got) "got command packet with incorrect checksum, expected=0x%02x, received=0x%02x"
 gdbstub_err_unexpected_runpkt(uint8_t ch) "unexpected packet (0x%02x) while target running"
 
-# softmmu.c
+# system.c
 gdbstub_hit_watchpoint(const char *type, int cpu_gdb_index, uint64_t vaddr) "Watchpoint hit, type=\"%s\" cpu=%d, vaddr=0x%" PRIx64 ""