Summary:
This patch disables large address space on OpenBSD. The motivation
for this is that OpenBSD does not support MAP_NORESERVE. The flag is supported
only for source code compatibility reasons but is otherwise completely ignored
by the OS and its mmap syscall.

Reviewers: austin, bgamari

Subscribers: thomie, erikd

Differential Revision: https://phabricator.haskell.org/D1411