[libc++][CMake] Always provide new/delete in libc++ unless specified otherwise

Summary:
Let's not try to be clever and detect it based on the libc++abi setting.
The only build that puts new/delete in libc++abi is Apple's and we set
this CMake option explicitly in both libc++ and libc++abi. Complicated
dependent options hurt, let's avoid them when possible.

Reviewers: phosek, EricWF

Subscribers: mgorny, christof, jkorous, dexonsmith, libcxx-commits

Tags: #libc

Differential Revision: https://reviews.llvm.org/D60797

llvm-svn: 358671