gdbserver: do not report btrace support if target does not announce it

Gdbserver unconditionally reports support for btrace packets.  Do not
report the support, if the underlying target does not say it supports
it.  Otherwise GDB would query the server with btrace-related packets
unnecessarily.

gdbserver/linux-low.cc

@@ -6712,6 +6712,12 @@ linux_process_target::qxfer_libraries_svr4 (const char *annex,
 
 #ifdef HAVE_LINUX_BTRACE
 
+bool
+linux_process_target::supports_btrace ()
+{
+  return true;
+}
+
 btrace_target_info *
 linux_process_target::enable_btrace (thread_info *tp,
 				     const btrace_config *conf)

gdbserver/linux-low.h

@@ -275,6 +275,8 @@ public:
   bool supports_agent () override;
 
 #ifdef HAVE_LINUX_BTRACE
+  bool supports_btrace () override;
+
   btrace_target_info *enable_btrace (thread_info *tp,
 				     const btrace_config *conf) override;
 

gdbserver/server.cc

@@ -2475,7 +2475,8 @@ handle_query (char *own_buf, int packet_len, int *new_packet_len_p)
       if (target_supports_agent ())
 	strcat (own_buf, ";QAgent+");
 
-      supported_btrace_packets (own_buf);
+      if (the_target->supports_btrace ())
+	supported_btrace_packets (own_buf);
 
       if (target_supports_stopped_by_sw_breakpoint ())
 	strcat (own_buf, ";swbreak+");

gdbserver/target.cc

@@ -694,6 +694,12 @@ process_stratum_target::supports_agent ()
   return false;
 }
 
+bool
+process_stratum_target::supports_btrace ()
+{
+  return false;
+}
+
 btrace_target_info *
 process_stratum_target::enable_btrace (thread_info *tp,
 				       const btrace_config *conf)

gdbserver/target.h

@@ -388,6 +388,9 @@ public:
   /* Return true if target supports debugging agent.  */
   virtual bool supports_agent ();
 
+  /* Return true if target supports btrace.  */
+  virtual bool supports_btrace ();
+
   /* Enable branch tracing for TP based on CONF and allocate a branch trace
      target information struct for reading and for disabling branch trace.  */
   virtual btrace_target_info *enable_btrace (thread_info *tp,