Remove ppc860, ppc750cl, ppc7450 insns from common ppc.

Remove ppc860, ppc750cl, ppc7450 insns from common ppc.

Back in the day support for these processors was added, we probably
didn't want to waste PPC_OPCODE bits on minor variations.  I've had a
complaint that disassembly of mfspr/mtspr was wrong for power8.  This
patch fixes that problem.

Note that since -m860/-m850/-m821 are new gas options enabling the
mpc8xx specific mfspr/mtspr variants it is possible that this change
will break some mpc8xx assembly code.  ie. you might need to modify
makefiles to pass -m860 to gas.

include/opcode/
	* ppc.h (PPC_OPCODE_750, PPC_OPCODE_7450, PPC_OPCODE_860): Define.
opcodes/
	* ppc-opc.c (PPC750, PPC7450, PPC860): Define using PPC_OPCODE_*.
	* ppc-dis.c (ppc_opts): Add 821, 850 and 860 entries.  Add
	PPC_OPCODE_7450 to 7450 entry.  Add PPC_OPCODE_750 to 750cl entry.
gas/
	* config/tc-ppc.c (md_show_usage): Add -m821, -m850, -m860.
	* doc/c-ppc.texi (PowerPC-Opts): Likewise.
gas/testsuite/
	* gas/ppc/titan.d: Correct mfmcsrr0 disassembly.