switch fatal error signal from T06 to S05

With the old signal, the first thing a GDB client would do post-mortem
is to send a resume-with-signal, which gdbstub doesn't currently
support.

This _could_ be suppressed by having users set `handle all nopass`, but
that's pretty no-beuno.

By simply reporting a standard TRAP signal, the GDB client won't do
anything too fancy, and users won't enounter the unfortunate
"ResumeWithSignalNotImplemented" error for no reason.

src/gdbstub_impl/mod.rs

@@ -180,12 +180,12 @@ impl<T: Target, C: Connection> GdbStubImpl<T, C> {
                             None
                         }
                         Err(Error::TargetError(e)) => {
-                            // unlike all other errors, which are "unrecoverable", there's a chance
-                            // that a target may be able to recover from a target-specific error. In
-                            // this case, we may as well report a SIGABRT stop reason, giving the
-                            // target a chance to open a "post-mortem" GDB session.
+                            // unlike all other errors which are "unrecoverable" in the sense that
+                            // the GDB session cannot continue, there's still a chance that a target
+                            // might want to keep the debugging session alive to do a "post-mortem"
+                            // analysis. As such, we simply report a standard TRAP stop reason.
                             let mut res = ResponseWriter::new(conn);
-                            res.write_str("T06")?; // SIGABRT
+                            res.write_str("S05")?;
                             res.flush()?;
                             return Err(Error::TargetError(e));
                         }