libs/libc:Support gdb rsp protocol

you can debug nuttx through any transport layer (serial port, network etc.),
currently supports the following functions:
1. Read and write registers
2. Read and write memory
3. Switch thread and read stack information

Future support plans:
1. Support breakpoint, watch point (requires architecture support).

related information:
https://sourceware.org/gdb/onlinedocs/gdb/Remote-Protocol.html

Signed-off-by: anjiahao <anjiahao@xiaomi.com>

libs/libc/gdbstub/Make.defs

@@ -0,0 +1,32 @@
+############################################################################
+# libs/libc/gdbstub/Make.defs
+#
+# Licensed to the Apache Software Foundation (ASF) under one or more
+# contributor license agreements.  See the NOTICE file distributed with
+# this work for additional information regarding copyright ownership.  The
+# ASF licenses this file to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance with the
+# License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+# License for the specific language governing permissions and limitations
+# under the License.
+#
+############################################################################
+
+ifeq ($(CONFIG_LIB_GDBSTUB),y)
+
+# Add the internal C files to the build
+
+CSRCS += lib_gdbstub.c
+
+# Add the userfs directory to the build
+
+DEPPATH += --dep-path gdbstub
+VPATH += :gdbstub
+
+endif