gdbserver: be stricter about qC

Don't match only the prefix of qC. Otherwise, compare-sections in
gdb will fail, due to qCRC returning a thread ID.

pyocd/gdbserver/gdbserver.py

@@ -954,7 +954,7 @@ def handle_query(self, msg):
                 # Must return an empty packet for an unrecognized qXfer.
                 return self.create_rsp_packet(b"")
 
-        elif query[0].startswith(b'C'):
+        elif query[0] == b'C':
             if not self.is_threading_enabled():
                 return self.create_rsp_packet(b"QC1")
             else: