Enable LLVM asan support for NetBSD/amd64

Summary:
Enable LLVM asan sanitizer for NetBSD/amd64.

Don't generate -ldl for dlopen(3)-like functions on NetBSD.
These features are available in libc on NetBSD.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, eugenis, kcc, dim

Reviewed By: dim

Subscribers: #clang, #sanitizers

Tags: #clang, #sanitizers

Differential Revision: https://reviews.llvm.org/D34960

llvm-svn: 307104