Fix/workaround for OS X truncated stacktraces taken by external tools

This patch is a proposed solution for https://code.google.com/p/address-sanitizer/issues/detail?id=375:

When the stacktraces are captured and printed by ASan itself, they are fine, but when the program has already printed the report (or is just printing it), capturing a stacktrace via other means is broken. "Other means" include OS X CrashReporter, debuggers or calling backtrace() within the program. For example calling backtrace() from a sanitizer_set_death_callback function prints a very truncated stacktrace.

Reviewed at http://reviews.llvm.org/D7103

llvm-svn: 226878