docs: Update the ShadowCallStack documentation.

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@353890 91177308-0d34-0410-b5e6-96231b3b80d8