[CMake] Use libtool for runtimes when building for Apple platform

LLVM CMake build already uses libtool instead of ar when building
for Apple platform and we should be using the same when building
runtimes. To do so, this change extracts the logic for finding
libtool into a separate file and then uses it from both the LLVM
build as well as the LLVM runtimes build.

Differential Revision: https://reviews.llvm.org/D62769

llvm-svn: 362313