[sanitizer] Implement TwoLevelByteMap and use it for the internal allocator on 64-bit.

Summary:
Implement TwoLevelByteMap and use it for the internal allocator on 64-bit.
This reduces bss on 64-bit by ~8Mb because we don't use FlatByteMap on 64-bits any more.

Dmitry, please check my understanding of atomics.

Reviewers: dvyukov

Reviewed By: dvyukov

CC: samsonov, llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D2259