Register NetBSD/x86_64 in MemorySanitizer.cpp

Summary:
Reuse the Linux new mapping as it is.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, eugenis, vitalybuka

Reviewed By: vitalybuka

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D41022

llvm-svn: 320219