[PATCH] [Sanitizers] Enable stack traces on recent versions of FreeBSD

[Viktor Kutuzov]

> On FreeBSD 10.0 _Unwind_Backtrace() behaves similarly to its Linux implementation when called from within a signal handler.

Wrong. It still return the handler's stack on SIGSEGV. Sorry for misleading. I'm going to prepare another patch that introduce SANITIZER_CAN_SLOW_UNWIND, enable stack traces on FreeBSD and rely on fast unwinding when called from within the Asan's SIGSEGV handler.

http://reviews.llvm.org/D6086