Tell msan that the arguments to the traceback functions are initialized,
in case the traceback functions are compiled with -fsanitize=memory.

Change-Id: I3ab0816604906c6cd7086245e6ae2e7fa62fe354
Reviewed-on: https://go-review.googlesource.com/24856
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>