[llvm-libtool-darwin] Add support for -arch_only

Add support for -arch_only option for llvm-libtool-darwin. This diff
also adds support for accepting universal files as input and flattening
them to create the required static library. Supports input universal
files contaning both Mach-O object files or archives.

Differences from cctools' libtool:
- `-arch_only` can be specified multiple times
- archives containing universal files are considered invalid (libtool
allows such archives)

Reviewed by jhenderson, smeenai

Differential Revision: https://reviews.llvm.org/D84770