PowerPC Improve support for Gekko & Broadway

This is a relatively straightforward patch to improve support for the
IBM Gekko and IBM Broadway processors.  Broadway is functionally
equivalent to the IBM 750CL, while Gekko's functionality is a subset
of theirs.  The patch simplifies this reality and adds -mgekko and
-mbroadway as aliases for -m750cl.  I didn't feel it was worth wasting
a PPC_OPCODE_* bit to differentiate Gekko.  The patch adds a number of
simplified mnemonics for special purpose register access.  Notably,
Broadway adds 4 additional IBAT and DBAT registers but these are not
assigned sequential SPR numbers.

gas/
	* config/tc-ppc.c (md_show_usage): Add -mgekko and -mbroadway.
	* doc/as.texi (Target PowerPC options): Add -mgekko and -mbroadway.
	* doc/c-ppc.texi (PowerPC-Opts): Likewise.
	* testsuite/gas/ppc/broadway.d,
	* testsuite/gas/ppc/broadway.s: New test for broadway.
	* testsuite/gas/ppc/ppc.exp: Run new test.
include/
	* opcode/ppc.h (PPC_OPCODE_750): Adjust comment.
opcodes/
	* ppc-dis.c (ppc_opts): Add -mgekko and -mbroadway.
	(powerpc_init_dialect): Handle bfd_mach_ppc_750.
	* ppc-opc.c (insert_sprbat, extract_sprbat): New functions to
	support disjointed BAT.
	(powerpc_operands): Allow extra bit in SPRBAT_MASK.  Add SPRGQR.
	(XSPRGQR_MASK, GEKKO, BROADWAY): Define.
	(powerpc_opcodes): Add 750cl extended mnemonics for spr access.

include/opcode/ppc.h

@@ -204,7 +204,7 @@ extern const unsigned int spe2_num_opcodes;
 /* Opcode is only supported by Power8 architecture.  */
 #define PPC_OPCODE_POWER8     0x1000000000ull
 
-/* Opcode is supported by ppc750cl.  */
+/* Opcode is supported by ppc750cl/Gekko/Broadway.  */
 #define PPC_OPCODE_750	      0x2000000000ull
 
 /* Opcode is supported by ppc7450.  */