[runtimes] Add a warning about the Standalone build being deprecated

We are removing support for the Standalone build altogether on the main
branch, so this is going to help give an additional heads up to users
that don't read our release notes.

Also, as a fly-by fix, fixup incorrect documentation for libcxxabi and
mention libunwind in our release notes.

Differential Revision: https://reviews.llvm.org/D119341