gallium/util: libunwind support

gallium/util: libunwind support

It's kinda sad that (a) we don't have debug_backtrace support on !X86
and that (b) we re-invent our own crude backtrace support in the first
place.  If available, use libunwind instead.  The backtrace format is
based on what xserver and weston use, since it is nice not to have to
figure out a different format.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Acked-by: Nicolai Hähnle <nicolai.haehnle@amd.com>