Stop intercepting forkpty(3) and openpty(3) on NetBSD

Summary:
forkpty(3) and openpty(3) are part of `-lutil` and we don't intend to reimplement
this system library in sanitizers. Everybody using these functions will need to use
a precompiled library against MSan or other desired sanitizer.

Restrict these functions to Linux-only.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, vitalybuka

Reviewed By: vitalybuka

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D43490

llvm-svn: 325585