Port msan_test.cc to NetBSD

Summary:
The changes allows building this file on NetBSD, mostly by
disabling the unsupported functions and adding OS-specific
system includes.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, kcc, vitalybuka

Reviewed By: vitalybuka

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D46712

llvm-svn: 332791