runtime: let ARM32 EABI personality function continue unwind when called from traceback

On ARM32 EABI, unlike other platforms, the personality function is
called during _Unwind_Backtrace (libgcc/unwind-arm-common.inc:581).
In this case, simply unwind the frame without returning any
handlers. Otherwise traceback will loop if there is a frame with
a defer on stack.

Change-Id: I8697b3cbc0b852422e8cd061bfef3803b74d4185
Reviewed-on: https://go-review.googlesource.com/c/155759
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/libgo/runtime/go-unwind.c b/libgo/runtime/go-unwind.c
index 388d7c7..58cb270 100644
--- a/libgo/runtime/go-unwind.c
+++ b/libgo/runtime/go-unwind.c
@@ -444,6 +444,9 @@
   switch (state & _US_ACTION_MASK)
     {
     case _US_VIRTUAL_UNWIND_FRAME:
+      if (state & _UA_FORCE_UNWIND)
+        /* We are called from _Unwind_Backtrace.  No handler to run.  */
+        CONTINUE_UNWINDING;
       actions = _UA_SEARCH_PHASE;
       break;