Use CMAKE_AR instead of the system default 'ar' for merging static libraries

Using the system default 'ar' might not be the right choice when
cross compiling.

Don't prepend the ar options by a dash, not all ar implementations
support that (llvm-ar doesn't).

Also pass the 's' option when creating the merged library, to create
an index.

Differential Revision: https://reviews.llvm.org/D37134

llvm-svn: 313122