Work around a glibc bug: backtrace() spuriously fails if

 - glibc is dynamically linked, and
 - libgcc_s is unavailable (for instance, another library is being used to
   provide the compiler runtime or libgcc is statically linked), and
 - the target is x86_64.

If we run backtrace() and it fails to find any stack frames, try using
_Unwind_Backtrace instead if available.

llvm-svn: 269992