[Bug sanitizer/79341] Many Asan tests fail on s390

[uweigand at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79341

--- Comment #48 from Ulrich Weigand <uweigand at gcc dot gnu.org> ---
s390(x) has -fasynchronous-unwind-tables on by default anyway, and .eh_frame
based DWARF unwinding is the only way to create stack backtraces that always
works.

However, I understood that asan deliberately doesn't want to use DWARF
unwinding for the the malloc/free case since it can be slow.  That's why Marcin
actually added -mbackchain to LLVM in the first place.  (We've had -mbackchain
in GCC forever, but it has defaulted to off for a very long time.)

I don't think we should switch to *always* using backchain unwinding in asan,
since system libraries on s390 will be built without backchain.  However,
switching -mbackchain on by default when building for asan might make sense.