[libc++] Use apple-install-libcxx.sh in the Apple/system CI job

That script is what we (need to) use to build libc++ for the system
configuration, so that's what we should test against. At some point
we may be able to fold all of that logic into the CMake build, and
when that happens the CI can go back to running CMake directly.

As a fly-by fix, stop mentioning x86_64 in the names of the Apple
jobs since they are not truly tied to any architecture.

Differential Revision: https://reviews.llvm.org/D111865