[compiler-rt] Add support for the dynamic shadow allocation

Summary:
This patch is adding the needed code to compiler-rt to support
dynamic shadow.

This is to support this patch:
  https://reviews.llvm.org/D23354

It's adding support for using a shadow placed at a dynamic address determined
at runtime.

The dynamic shadow is required to work on windows 64-bits.

Reviewers: rnk, kcc, vitalybuka

Subscribers: kubabrecka, dberris, llvm-commits, chrisha

Differential Revision: https://reviews.llvm.org/D23363

llvm-svn: 281909