Do not link with -ldl in NetBSD/ASan tests

Summary:
The dlopen(3) features on NetBSD are in libc.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, vitalybuka, kcc

Reviewed By: vitalybuka

Subscribers: kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D46713

llvm-svn: 332790