[runtimes] Always configure libc++abi before libc++

That makes it possible to reuse libc++abi targets from the libc++
configuration, which is necessary to allow major CMake simplifications.
As a fly-by fix, we also unify how compiler-rt ordering is handled so
it matches how libc++ and libc++abi are handled (compiler-rt always
ends up first).

Differential Revision: https://reviews.llvm.org/D120719