Add -mcpu=future support.

Add -mcpu=future support.

This patch adds support for -mcpu=future.  At the current time, no new
instructions are implemented.  The tuning for the future processor is exactly
the same as for power10.

If -mcpu=future is used, the macro _ARCH_PWR_FUTURE will be defined.

In order to use -mcpu=future, you will need an assembler that supports the
-mfuture option.

2024-02-29  Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config.gcc (powerpc*-*-*, rs6000-*-*): Add support for a potential
	future processor.
	* config/rs6000/aix71.h (ASM_CPU_SPEC): Likewise.
	* config/rs6000/aix72.h (ASM_CPU_SPEC): Likewise.
	* config/rs6000/aix73.h (ASM_CPU_SPEC): Likewise.
	* config/rs6000/rs6000-c.cc (rs6000_target_modify_macros): Define
	_ARCH_PWR_FUTURE if -mcpu=future.
	* config/rs6000/rs6000-cpus.def (ISA_FUTURE_MASKS_SERVER): New macro.
	(POWERPC_MASKS): Add support for -mcpu=future.
	(future cpu): Add future cpu.
	* config/rs6000/rs6000-opts.h (PROCESSOR_FUTURE): New macro.
	* config/rs6000/rs6000-tables.opt: Regenerate.
	* config/rs6000/rs6000.cc (rs6000_machine_from_flags): If -mcpu=future,
	emit .machine future.
	(rs6000_opt_masks): Add support for the future ISA bits.
	* config/rs6000/rs6000.h (ASM_CPU_SPEC): Add support for the future
	processor.
	* config/rs6000/rs6000.opt (-mfuture): New internal ISA bit for a
	potential future cpu.
	* doc/invoke.texi (PowerPC options): Document -mcpu=future.