[builtins] Use libtool for builtins when building for Apple platform

compiler-rt already uses libtool instead of ar when building for
Apple platform, but that's not being used when builtins are being
built separately e.g. as part of the runtimes build. This change
extracts the logic setting up libtool into a separate file and uses
it from both the compiler-rt and standalone builtins build.

Differential Revision: https://reviews.llvm.org/D62820

llvm-svn: 362466