[libc++abi] Remove unistd.h include
[libc++abi] Remove unistd.h include

This was originally there for the _POSIX_THREADS define, to detect the
presence of pthreads. That went away with the externalized threading
support, so the include can go away too.

config.h is now completely empty. A follow-up commit will remove it
entirely.

git-svn-id: 6a9f6578bdee8d959f0ed58970538b4ab6004734@299087 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi