Add 64-bit and multihart support

This is a major rewrite of the RISC-V v0.13 OpenOCD port.  This
shouldn't have any meaningful effect on the v0.11 support, but it does
add generic versions of many functions that will allow me to later
refactor the v0.11 support so it's easier to maintain both ports.  This
started as an emergency feature branch and went on for a long time, so
it's all been squashed down into one commit so there isn't a big set of
broken commits lying around.  The changes are:

 * You can pass "-rtos riscv" to the target in OpenOCD's configuration
   file, which enables multi-hart mode.  This uses OpenOCD's RTOS
   support to control all the harts from the debug module using commands
   like "info threads" in GDB.  This support is still expermental.

 * There is support for RV64I, but due to OpenOCD limitations we only
   support 32-bit physical addresses.  I hope to remedy this by rebasing
   onto the latest OpenOCD release, which I've heard should fix this.

 * This matches the latest draft version of the RISC-V debug spec, as of
   April 26th.  This version fixes a number of spec bugs and should be
   close to the final debug spec.

src/server/gdb_server.c

@@ -2445,6 +2445,13 @@ static int gdb_v_packet(struct connection *connection,
 	struct gdb_service *gdb_service = connection->service->priv;
 	int result;
 
+	struct target *target = get_target_from_connection(connection);
+	if (target->rtos != NULL && target->rtos->gdb_v_packet != NULL) {
+		int out = target->rtos->gdb_v_packet(connection, packet, packet_size);
+		if (out != GDB_THREAD_PACKET_NOT_CONSUMED)
+			return out;
+	}
+
 	/* if flash programming disabled - send a empty reply */
 
 	if (gdb_flash_program == 0) {
@@ -2643,7 +2650,7 @@ static void gdb_log_callback(void *priv, const char *file, unsigned line,
 	gdb_output_con(connection, string);
 }
 
-static void gdb_sig_halted(struct connection *connection)
+void gdb_sig_halted(struct connection *connection)
 {
 	char sig_reply[4];
 	snprintf(sig_reply, 4, "T%2.2x", 2);
@@ -3195,3 +3202,9 @@ int gdb_register_commands(struct command_context *cmd_ctx)
 	gdb_port_next = strdup("3333");
 	return register_commands(cmd_ctx, NULL, gdb_command_handlers);
 }
+
+void gdb_set_frontend_state_running(struct connection *connection)
+{
+	struct gdb_connection *gdb_con = connection->priv;
+	gdb_con->frontend_state = TARGET_RUNNING;
+}

src/server/gdb_server.h

@@ -45,6 +45,9 @@ static inline struct target *get_target_from_connection(struct connection *conne
 	return gdb_service->target;
 }
 
+void gdb_set_frontend_state_running(struct connection *connection);
+void gdb_sig_halted(struct connection *connection);
+
 #define ERROR_GDB_BUFFER_TOO_SMALL (-800)
 #define ERROR_GDB_TIMEOUT (-801)