[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

compiler-rt/lib/sanitizer_common/sanitizer_stacktrace.cc

@@ -92,6 +92,8 @@ void BufferedStackTrace::FastUnwindStack(uptr pc, uptr bp, uptr stack_top,
         !IsAligned((uptr)caller_frame, sizeof(uhwptr)))
       break;
     uhwptr pc1 = caller_frame[2];
+#elif defined(__s390__)
+    uhwptr pc1 = frame[14];
 #else
     uhwptr pc1 = frame[1];
 #endif