[libunwind] minimalistic x86 support

[David Mosberger]

The latest bitkeeper tree has minimalistic support for x86.  It only
works with functions that maintain a frame-pointer and is primiarily
intended to verify that the build-environment etc. is sane w.r.t.
multiple target architectures.  Having said that, tests/bt works
perfectly fine, so I guess libunwind is now officially
multi-architecture. ;-)

My plan is to finish (and test) the ia64 support next and then add
some documentation.

As usual, comments and contributions welcome.

Enjoy,

        --david