[runtimes] Use the new "runtimes" build by default and deprecate other builds

This commit makes the new "runtimes" build (with <monorepo>/runtimes as
the root of the CMake invocation) the default way of building libc++.
The other supported way of building libc++ is the "bootstrapping" build,
where `<monorepo>/llvm` is used as the root of the CMake invocation.

All other ways of building libc++ are deprecated effective immediately.
There should be no use-case for building libc++ that isn't supported by
one of these two builds, and the two new builds work on all environments
and are lightweight. They will also make it possible to greatly simplify
the build infrastructure of the runtimes, which is currently way too
convoluted.

Differential Revision: https://reviews.llvm.org/D111356