Move gdbsupport to the top level

[Tom Tromey <tom at tromey dot com>]

This patch moves the gdbsupport directory to the top level.  This is
the next step in the ongoing project to move gdbserver to the top
level.

The bulk of this patch was created by "git mv gdb/gdbsupport gdbsupport".

This patch then adds a build system to gdbsupport and wires it into
the top level.  Then it changes gdb to use the top-level build.

gdbserver, on the other hand, is not yet changed.  It still does its
own build of gdbsupport.

I wasn't able to send this through the buildbot.  I did test it on
x86-64 Fedora 29.  If you want to try it, it is on the branch
submit/move-gdbsupport-to-top in my github.


This patch was also too big to send the usual way, so I have compressed
it and added it as an attachment.  Sorry about that.

Tom

Attachment: 0001-Move-gdbsupport-to-the-top-level.patch.xz
Description: the patch