Don't forward all assembler arguments untouched to -cc1as

Use the same filtering for assembly arguments to -cc1as as we do for
-cc1, this allows a consistent (& more useful) diagnostic experience for
users (rather than getting an error from -cc1as (which a user shouldn't
really be thinking about) about --foo, they get an error from clang
about --foo in -Wa,)

I'm sort of surprised by the separation of -cc1as & the separate
argument handling, etc, but at least this removes a little bit of the
duplication.