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Update GDB and OpenOCD to work together, finally (#482)

GDB for non-Linux systems was built w/o expat which caused odd behavior
under Windows and other systems (i.e. breakpoints not working, etc.)
New toolchain manually builds cross-compiled libexpat and ensures it is
used, fixing the issue.

Windows OpenOCD binaries now come from manually built and tested copies
(using a real Windows system).

Fixes #478
Fixes #457
Fixes #456
and probably others...