Prefer ranlib mode over ar mode.

For CloudABI's toolchain I have a symlink that goes from <target>-ar and
<target>-ranlib to LLVM's ar binary, to mimick GNU Binutils' naming
scheme. The problem is that if we're targetting ARM64, the name of the
ranlib executable is aarch64-unknown-cloudabi-ranlib. This already
contains the string "ar".

Let's move the "ranlib" test above the "ar" test. It's not that likely
that we're going to see operating systems or harwdare architectures that
are called "ranlib".

Reviewed by:	rafael
Differential Revision:	http://reviews.llvm.org/D14123

llvm-svn: 251413