[runtimes] Use llvm-libtool-darwin for runtimes build

It's full featured now and we can use it for the runtimes build instead
of relying on an external libtool, which means the CMAKE_HOST_APPLE
restriction serves no purpose either now. Restrict llvm-lipo to Darwin
targets while I'm here, since it's only needed there.

Reviewed By: phosek

Differential Revision: https://reviews.llvm.org/D86367