Fix detection of backtrace() availability on FreeBSD

On FreeBSD backtrace is not part of libc and depends on libexecinfo
being available. Instead of using manual checks we can use the builtin
CMake module FindBacktrace.cmake to detect availability of backtrace()
in a portable way.

Patch By:	Alex Richardson
Differential Revision:	https://reviews.llvm.org/D27143

llvm-svn: 300062