Add NetBSD ASAN shadow mapping for x86-64

Summary:
The maximal virtual address on NetBSD/amd64 is 0x7f7ffffff000.
Define shadow offset 0x400000000000 (1ULL << 46).

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, vitalybuka, filcab, fjricci, kcc, eugenis, pcc

Reviewed By: kcc

Subscribers: llvm-commits, kubamracek, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D36587

llvm-svn: 311937