Fix encoding of CPU32 tbls insns

[Ben Elliston]

> gas/testsuite/:
> 	* gas/m68k/cpu32.[sd]: New test.
> 	* gas/m68k/all.exp: Run it.
> 
> opcodes/:
> 	* m68k-opc.c: Fix encoding of signed bit in the cpu32 tbls insns.

OK, thanks.

Cheers,
Ben