We need unwind tables to be precise at the instruction level, so they
can be used for accurate stack unwinding (e.g. backtrace()) from
asynchronous events like signals. Slackspot depends on this ability.

Failure to do so may cause applications to report incomplete
backtraces with glibc's backtrace(), or even crash over libunwind, see
http://xenomai.org/pipermail/xenomai/2017-October/037763.html

Enable -fasynchronous-unwind-tables for building the Xenomai libraries
and the user applications (via xeno-config --cflags).

Report and suggestion for a fix by Andreas Glatz
<andi.glatz@gmail.com>.