Register NetBSD/i386 in asan_mapping.h

Summary:
Introduce kNetBSD_ShadowOffset32 and document
NetBSD/i386 (hosted on amd64 kernel) process virtual
address space ranges.

Sponsored by <The NetBSD Foundation>

Reviewers: joerg, kcc, vitalybuka

Reviewed By: vitalybuka

Subscribers: kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D46708

llvm-svn: 332027