[hwasan] LLVM-level flags for linux kernel-compatible hwasan instrumentation.

Summary:
-hwasan-mapping-offset defines the non-zero shadow base address.
-hwasan-kernel disables calls to __hwasan_init in module constructors.
Unlike ASan, -hwasan-kernel does not force callback instrumentation.
This is controlled separately with -hwasan-instrument-with-calls.

Reviewers: kcc

Subscribers: srhines, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D42141

llvm-svn: 322785