[CMake] [Darwin] Use libtool instead of ar && ranlib

Summary: Using libtool instead of ar and ranlib on Darwin shaves a minute off my clang build. This is because on Darwin libtool is optimized to give hints to the kernel about filesystem interactions that allow it to be faster.

Reviewers: bogner, pete

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19611

llvm-svn: 267930