gas: scfidw2gen: new functionality to prepapre for SCFI

Since the option --scfi=inline still needs to be worked out, the
implementation in scfidw2gen remains in the form it currently is.  This
implementation needs to be cleaned up thereafter.

gas/
        * Makefile.am: Add new files to GAS_CFILES and HFILES.
        * Makefile.in: Likewise.
        * scfidw2gen.c: New file.
        * scfidw2gen.h: New file.