llvm-ar: Start adding support for mri scripts.

I was quiet surprised to find this feature being used. Fortunately the uses
I found look fairly simple. In fact, they are just a very verbose version
of the regular ar commands.

Start implementing it then by parsing the script and setting the command
variables as if we had a regular command line.

This patch adds just enough support to create an empty archive and do a bit
of error checking. In followup patches I will implement at least addmod
and addlib.

From the description in the manual, even the more general case should not
be too hard to implement if needed. The features that don't map 1:1 to
the simple command line are

* Reading from multiple archives.
* Creating multiple archives.