Attempt to fix the FreeBSD build, disable multiprocessing

Speculative quick fix based on clang-X86_64-freebsd output:

  File "/usr/local/lib/python2.6/multiprocessing/synchronize.py", line 33, in <module>
    " function, see issue 3770.")
ImportError: This platform lacks a functioning sem_open implementation, therefore, the required synchronization primitives needed will not function, see issue 3770.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193457 91177308-0d34-0410-b5e6-96231b3b80d8