Define OFF_T as 64-bit integer on NetBSD

Summary:
All 32 and 64 bit NetBSD platforms define off_t as 64-bit integer.

Part of the code inspired by the original work on libsanitizer in GCC 5.4 by Christos Zoulas.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, filcab, kcc, vitalybuka

Reviewed By: vitalybuka

Subscribers: emaste, kubamracek, llvm-commits

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D35553

llvm-svn: 310349