lib: add libunwind support for backtraces

lib: add libunwind support for backtraces

libunwind provides an alternate to backtrace() for printing out the call
stack of a particular location.  It doesn't use the frame pointer, it
goes by the DWARF debug info.  In most cases the traces have exactly the
same information, but there are some situations where libunwind traces
are better.

(On some platforms, the libc backtrace() also uses the DWARF debug info
[e.g.: ARM backtraces are impossible without it] but this is not the
case everywhere, especially not on BSD libexecinfo.)

Signed-off-by: David Lamparter <equinox@diac24.net>