Add support for FreeBSD

Port the OpenMP runtime to FreeBSD along with associated build system changes.

Also begin to generalize affinity capabilities so they aren't tied explicitly
to Windows and Linux.

The port builds with stock clang and gmake and has no additional runtime
dependencies.

All but a handful of the validation suite tests are now passing on FreeBSD 10
x86_64.

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@202478 91177308-0d34-0410-b5e6-96231b3b80d8