docs: Update the ShadowCallStack documentation.

- Remove most of the discussion of the x86_64 implementation;
  link to an older version of the documentation for details of
  that implementation.
- Add description of the compatibility and security issues discovered
  during the development of the aarch64 implementation for Android.

Differential Revision: https://reviews.llvm.org/D58105

llvm-svn: 353890