Fix cross-gdb build on certain Linux distros

[flannelhead]

Here's a one-liner patch to crosstool-config-overrides to mitigate problems with building cross-gdb on certain platforms.

--with-guile=guile-2.0: some distros (like Arch) have guile 2.2 as the default version, but gdb can't be built against that (upstream bug report). This flag tells gdb to build against guile-2.0.

--disable-tui: some distros provide ncurses and tinfo as separate packages. Apparently gdb doesn't detect this properly and fails to link against tinfo. These libraries are only required for the TUI, so disable it until this is fixed. crosstool-NG bug report

[riban-bw]

--disable-tui is required for cygwin32 builds. It would be advantageous to add this patch.