gdb, gdbserver, gdbsupport: add .gitattributes files

Create .gitattributes files in gdb/, gdbserver/, and gdbsupport/.

The files specify cpp-style diffs for .h and .c files.  This is
particularly helpful if a class in a header file is modified.
For instance, if the `stop_requested` field of `thread_info` in
gdb/gdbthread.h is modified, we get the following diff with
'git diff' (using git version 2.17.1):

   @@ -379,7 +379,7 @@ public:
      struct target_waitstatus pending_follow;

      /* True if this thread has been explicitly requested to stop.  */
   -  int stop_requested = 0;
   +  bool stop_requested = 0;

      /* The initiating frame of a nexting operation, used for deciding
         which exceptions to intercept.  If it is null_frame_id no

Note that the context of the change shows up as 'public:'; not so
useful.  With the .gitattributes file, we get:

   @@ -379,7 +379,7 @@ class thread_info : public refcounted_object
      struct target_waitstatus pending_follow;

      /* True if this thread has been explicitly requested to stop.  */
   -  int stop_requested = 0;
   +  bool stop_requested = 0;

      /* The initiating frame of a nexting operation, used for deciding
         which exceptions to intercept.  If it is null_frame_id no

The context is successfully shown as 'class thread_info'.

This patch creates a .gitattributes file per each of gdb, gdbserver,
and gdbsupport folders.  An alternative would be to define the
attributes in the root folder -- this would impact all the top-level
folders, though.  I opted for the more conservative approach.

gdb/ChangeLog:
2020-03-05  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* .gitattributes: New file.

gdbserver/ChangeLog:
2020-03-05  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* .gitattributes: New file.

gdbsupport/ChangeLog:
2020-03-05  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>

	* .gitattributes: New file.

gdb/.gitattributes

@@ -0,0 +1,7 @@
+# -*- conf -*-
+
+# Use cpp-style diffs for .h and .c files.  This is useful
+# if you modify classes defined in those files.
+
+*.h	diff=cpp
+*.c	diff=cpp

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2020-03-05  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>
+
+	* .gitattributes: New file.
+
 2020-03-04  Tom Tromey  <tom@tromey.com>
 
 	* symmisc.c (print_symbol_bcache_statistics)

gdbserver/.gitattributes

@@ -0,0 +1,6 @@
+# -*- conf -*-
+
+# Use cpp-style diffs for .h files.  This is useful
+# if you modify classes defined in a header file.
+
+*.h	diff=cpp

gdbserver/ChangeLog

@@ -1,3 +1,7 @@
+2020-03-05  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>
+
+	* .gitattributes: New file.
+
 2020-03-02  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* remote-utils.cc (prepare_resume_reply): Add ability to convert T

gdbsupport/.gitattributes

@@ -0,0 +1,6 @@
+# -*- conf -*-
+
+# Use cpp-style diffs for .h files.  This is useful
+# if you modify classes defined in a header file.
+
+*.h	diff=cpp

gdbsupport/ChangeLog

@@ -1,3 +1,7 @@
+2020-03-05  Tankut Baris Aktemur  <tankut.baris.aktemur@intel.com>
+
+	* .gitattributes: New file.
+
 2020-03-03  Simon Marchi  <simon.marchi@efficios.com>
 
 	* Makefile.in: Re-generate.