Rename gc68060.lib to gc.lib, cord/cord68060.lib to cord.lib in SMakefile

Rename gc68060.lib to gc.lib, cord/cord68060.lib to cord.lib in SMakefile

This is to match the behavior of the CMake script.

* SMakefile.amiga (clean): Remove cord/*.lib, cord/*.lnk.
* SMakefile.amiga (gctest, cordtest, gc$(CPU).lib): Rename gc$(CPU).lib
to gc.lib.
* SMakefile.amiga (cordtest, cord/cord$(CPU).lib): Rename
cord/cord$(CPU).lib to cord.lib.
* SMakefile.amiga (cordtest): Use $(LINKER) instead of slink.
* SMakefile.amiga (cord.lib): Use $(LIBER) instead of oml.
* SMakefile.amiga (cord/cordbscs.o, cord/cordprnt.o, cord/cordxtra.o,
cord/cordtest.o): Use $(CC) instead of sc.