Turn on -mpcrel by default for -mcpu=future

Turn on -mpcrel by default for -mcpu=future

2020-04-25  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/linux64.h (PCREL_SUPPORTED_BY_OS): Define to
	enable PC-relative addressing for -mcpu=future.
	* config/rs6000/rs6000-cpus.def (ISA_FUTURE_MASKS_SERVER): Move
	after OTHER_FUTURE_MASKS.  Use OTHER_FUTURE_MASKS.
	* config/rs6000/rs6000.c (PCREL_SUPPORTED_BY_OS): If not defined,
	suppress PC-relative addressing.
	(rs6000_option_override_internal): Split up error messages
	checking for -mprefixed and -mpcrel.  Enable -mpcrel if the target
	system supports it.