[llvm-libtool-darwin] Allow flattening archives

Add support for flattening archives while creating static libraries.
Hence, can now pass archives as input in addition to Mach-O binaries.
Furthermore, archives themselves must only conatain Mach-O binaries. As
per cctools' libtool's behavior, llvm-libtool-darwin does not flatten
archives recursively.

Reviewed by alexshap, smeenai, jhenderson

Differential Revision: https://reviews.llvm.org/D83520