Avoid usage of F_DUPFD_CLOEXEC where not available (e.g. kfreebsd*)

Summary:
kfreebsd doesn't have F_DUPFD_CLOEXEC, so use it conditionally.

Author: Emilio Pozuelo Monfort <pochu@debian.org>
Author: Petr Salinger <Petr.Salinger@seznam.cz>
Author: Gianfranco Costamagna

Reviewers: emaste

Subscribers: emaste

Differential Revision: http://reviews.llvm.org/D12429

llvm-svn: 246294