Allow PPC users to select which PPC/RS6000 variant they're debugging

Allow PPC users to select which PPC/RS6000 variant they're debugging
at run-time.  At the moment, the only thing this affects is the
set of registers visible.
* config/rs6000/tm-rs6000.h (REGISTER_NAME): Define this as a call
to the function rs6000_register_name.
(rs6000_register_name): Include extern decl.
(NUM_REGS): Bump to 183.  What's the right way to do this?
(FIRST_UISA_SP_REGNUM, LAST_UISA_SP_REGNUM): Renamed from
FIRST_SP_REGNUM, LAST_SP_REGNUM.
(REGISTER_BYTES): Recompute this.
* rs6000-tdep.c: Renamed all uses of FIRST_SP_REGNUM and
LAST_SP_REGNUM to FIRST_UISA_SP_REGNUM and LAST_UISA_SP_REGNUM, with
some concomitant formatting changes.
#include "gdbcmd.h", so we can define commands here.
(struct variant): New structure.
(COMMON_UISA_REG_NAMES, PPC_UISA_SPR_NAMES, PPC_SEGMENT_REG_NAMES,
PPC_32_OEA_SPR_NAMES, num_registers): New macros.
(register_names_rs6000, register_names_uisa, register_names_403,
register_names_403GC, register_names_505, register_names_860,
register_names_601, register_names_602, register_names_603,
register_names_604, register_names_750, variants): New variables.
(rs6000_register_name, install_variant, find_variant_by_name,
install_variant_by_name, list_variants, show_current_variant,
set_processor, show_processor): New functions.
(_initialize_rs6000_tdep): Define new commands `set processor' and
`show processor', and call install_variant_by_name to set the
default variant.
* rs6000-nat.c: Renamed all uses of FIRST_SP_REGNUM and
LAST_SP_REGNUM to FIRST_UISA_SP_REGNUM and LAST_UISA_SP_REGNUM, with
some concomitant formatting changes.
* configure.in: Accept the `--with-cpu' flag, to specify a default
processor variant.
* acconfig.h: Provide a blurb for TARGET_CPU_DEFAULT, which is set
by configure's `--with-cpu' flag.
* config.in, configure: Regenerated.