Move gdbsupport to the top level

[Tom Tromey]

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

Sergio and Gary made some fixes to the buildbot and I was able to send
this through.  That showed that this doesn't build on mingw; so I am
going to fix this sometime soon and then send another version of this
patch.

I don't anticipate very extensive changes, so feel free to look at this
patch if you want.

Tom