[libcxx-dev] Clarifying the supported ways to build libc++, libc++abi and libunwind

[Raul Tambre via libcxx-dev]

 >1. What is a "Standalone build"? What does it enable that a normal 
monorepo build can't?

A build where only a specific project is compiled, i.e. not as part of LLVM.

I build a LLVM toolchain for development at the company I work for. I 
only need to crosscompile compiler-rt, libcxx, libcxxabi, libunwind for 
the AArch64 target machine. Standalone builds save a lot of build time 
and simplify the setup because I don't have to crosscompile the whole 
LLVM, which is useless to me.

Standalone builds also simplify packaging, because I can install each 
project into its own directory.

I would very much like this to stay possible and will gladly fix issues 
found with this setup.

 >2. What is the "Runtimes" build? How does it work, what is it used 
for, and what does it expect from libc++/libc++abi/libunwind?

The above process of building the runtime libraries and packaging them.
I also statically link compiler-rt, libcxxabi and libunwind into libcxx.