Fix copyright notices

gas/COPYING

gas/app.c

gas/as.c

gas/as.h

gas/asintl.h

gas/atof-generic.c

gas/bignum-copy.c

gas/bignum.h

gas/bit_fix.h

gas/cgen.c

gas/cgen.h

gas/cond.c

gas/config/aout_gnu.h

gas/config/atof-ieee.c

gas/config/atof-tahoe.c

gas/config/atof-vax.c

gas/config/itbl-mips.h

gas/config/m68k-parse.h

gas/config/m68k-parse.y

gas/config/m88k-opcode.h