[Sanitizer] Fix segfaults during unwinding on SystemZ

[iii]

Summary

Every now and then SystemZ programs built with ASan crash with

ERROR: AddressSanitizer: stack-overflow on address 0x040000000000

for no apparent reason. The problem is that
BufferedStackTrace::UnwindFast() is specialized for SystemZ: it takes
register 14 from the frame, however, IsValidFrame() is not
specialized, and does not guarantee that frame[14] is going to be a
valid memory access.

Fix by introducing per-arch kFrameSize and using it in IsValidFrame().

[uweigand]

Yes, this makes sense to me as well. Patch LGTM.

compiler-rt/lib/sanitizer_common/sanitizer_stacktrace.h

private:
   uptr LocatePcInTrace(uptr pc);
 
   BufferedStackTrace(const BufferedStackTrace &) = delete;
   void operator=(const BufferedStackTrace &) = delete;
 
   friend class FastUnwindTest;
 };
 
+#if defined(__s390x__)
+static const uptr kFrameSize = 160;
+#elif defined(__s390__)
+static const uptr kFrameSize = 96;
+#else
+static const uptr kFrameSize = 2 * sizeof(uhwptr);
+#endif
+
 // Check if given pointer points into allocated stack area.
 static inline bool IsValidFrame(uptr frame, uptr stack_top, uptr stack_bottom) {
-  return frame > stack_bottom && frame < stack_top - 2 * sizeof (uhwptr);
+  return frame > stack_bottom && frame < stack_top - kFrameSize;
 }
 
 }  // namespace __sanitizer
 
 // Use this macro if you want to print stack trace with the caller
 // of the current function in the top frame.
 #define GET_CALLER_PC_BP \
   uptr bp = GET_CURRENT_FRAME();              \