Enable standalone-debug by default on FreeBSD

It was set by default on Darwin in r198655.  The same usability issues
with DTrace and LLDB apply to FreeBSD, so set it by default there too.

rdar://problem/15758808
http://llvm.org/pr19676

Differential Revision: http://reviews.llvm.org/D3448

llvm-svn: 208310