[HWASan] Introduce non-zero based and dynamic shadow memory (compiler-rt).

[HWASan] Introduce non-zero based and dynamic shadow memory (compiler-rt).

Summary:
Retire the fixed shadow memory mapping to avoid conflicts with default
process memory mapping (currently manifests on Android).

Tests on AArch64 show <1% performance loss and code size increase,
making it possible to use dynamic shadow memory by default.

For the simplicity and unifirmity sake, use dynamic shadow memory mapping
with base address accessed via ifunc resolver on all supported platforms.

Keep the fixed shadow memory mapping around to be able to run
performance comparison tests later.

Complementing D45840.

Reviewers: eugenis

Subscribers: srhines, kubamracek, dberris, mgorny, kristof.beyls, delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D45847

llvm-svn: 330474