[sanitizer] [SystemZ] Fix stack traces.

On s390, the return address is in %r14, which is saved 14 words from
the frame pointer.

Unfortunately, there's no way to do a proper fast backtrace on SystemZ
with current LLVM - the saved %r15 in fixed-layout register save
area points to the containing frame itself, and not to the next one.
Likewise for %r11 - it's identical to %r15, unless alloca is used
(and even if it is, it's still useless).  There's just no way to
determine frame size / next frame pointer.  -mbackchain would fix that
(and make the current code just work), but that's not yet supported
in LLVM.  We will thus need to XFAIL some asan tests
(Linux/stack-trace-dlclose.cc, deep_stack_uaf.cc).

Differential Revision: http://reviews.llvm.org/D18895

llvm-svn: 266371