Fix the build of thread.cpp on CloudABI.
Fix the build of thread.cpp on CloudABI.

CloudABI does provide unistd.h, but doesn't define __unix__. We need to
include this header file to make hardware_concurrency work.


git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@294832 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx