Define NetBSD/amd64 ASAN Shadow Offset

Summary:
Catch up after compiler-rt changes and define kNetBSD_ShadowOffset64
as (1ULL << 46).
 
Sponsored by <The NetBSD Foundation>

Reviewers: kcc, joerg, filcab, vitalybuka, eugenis

Reviewed By: eugenis

Subscribers: llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D37234

llvm-svn: 311941