gdb, gdbserver, gdbsupport: remove includes of early headers

Now that defs.h, server.h and common-defs.h are included via the
`-include` option, it is no longer necessary for source files to include
them.  Remove all the inclusions of these files I could find.  Update
the generation scripts where relevant.

Change-Id: Ia026cff269c1b7ae7386dd3619bc9bb6a5332837
Approved-By: Pedro Alves <pedro@palves.net>