Intercept accept4() on NetBSD

Summary:
The accept4() function first appeared in NetBSD 8.0.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, vitalybuka

Reviewed By: vitalybuka

Subscribers: llvm-commits, srhines, kubamracek, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D42046

llvm-svn: 323018