Add import facility to cmm parser

Add import facility to cmm parser

The cmm parser classified all unknown variables as imported. This
behaviour is not changed with this patch. Explicitly imported
variables get a different internal label type, namely ForeignLabel
instead of RtsLabel. In PIC compilation, the NCG creates dynamic
access for ForeignLabel, while RtsLabel are not treated as PIC when
-package rts. Hence, putting explicit imports into cmm files enables
the NCG to distinguish between RTS-external and RTS-internal symbols,
and create the appropriate code.