libunwind: document tested FreeBSD configs and sort OS list

libunwind is known to work on FreeBSD i386, amd64 (x86_64) and arm64.
It is the unwinder provided by the base system on all of those
architectures.

While here sort the OS list.

Differential Revision:	https://reviews.llvm.org/D38900

llvm-svn: 315814