[libc++abi][ci] Add a from-scratch config for libc++abi on Apple/system

[libc++abi][ci] Add a from-scratch config for libc++abi on Apple/system

I came across an issue where since we build the library for Apple with
the install name directory being /usr/lib, which means that if we don't
run the tests with DYLD_LIBRARY_PATH, we'll end up loading the
system-provided libc++abi when running the tests. That wreaks havoc.

Instead of fixing it in the legacy config file, this commit introduces
an Apple libc++abi config file that does the right thing.

Differential Revision: https://reviews.llvm.org/D111279