Disable detection of on_exit()/TSan on NetBSD

Summary:
NetBSD does not ship with on_exit() function.

Introduce TSAN_MAYBE_INTERCEPT_ON_EXIT.
It looks like this addition fixes build for Darwin.

Sponsored by <The NetBSD Foundation>

Reviewers: vitalybuka, joerg, eugenis, dvyukov, kcc

Reviewed By: vitalybuka

Subscribers: llvm-commits, kubamracek, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D39617

llvm-svn: 317361