Avoid O_CLOEXEC to allow building on older Linux (RHEL5)

Summary:
See https://github.com/google/sanitizers/issues/1253.

Small patch to enable compilation on (ancient) Red Hat Enterprise Linux 5.

Reviewers: kcc, vitalybuka

Reviewed By: vitalybuka

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D80648