[sanitizer] Use mmap to zero-fill large shadow regions.

This is covered by existing ASan test.
This does not change anything for TSan by default (but provides a flag to 
change the threshold size).
Based on a patch by florent.bruneau here:
  https://code.google.com/p/address-sanitizer/issues/detail?id=256



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@201400 91177308-0d34-0410-b5e6-96231b3b80d8