OpenBSD sanitizer common, define RLIMIT_AS constant

Summary: define RLIMIT_AS constant until it s defined in the future

Patch by: David Carlier

Reviewers: krytarowski, vitalybuka

Reviewed By: vitalybuka

Subscribers: kubamracek, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D44068

llvm-svn: 326768