Remove legacy CMake targets for libcxx and libcxxabi

We've been meaning to remove those targets for a while, and the fix is
simple enough cause they're all just aliases to other targets.

This is a re-application of f383fb40, wich was reverted in 04d48111
because the build bots had not been updated yet. The build bot configurations
have now been updated not to use the deprecated targets, and I verified
that they were using the non-deprecated targets, so we should be good
unless I missed a bot.

Differential Revision: https://reviews.llvm.org/D76104