aarch64: add 'AARCH64_EXTRA_TUNE_FULLY_PIPELINED_FMA'

This patch adds a new tuning option
'AARCH64_EXTRA_TUNE_FULLY_PIPELINED_FMA', to consider fully
pipelined FMAs in reassociation. Also, set this option by default
for Ampere CPUs.

gcc/ChangeLog:

	* config/aarch64/aarch64-tuning-flags.def
	(AARCH64_EXTRA_TUNING_OPTION): New tuning option
	AARCH64_EXTRA_TUNE_FULLY_PIPELINED_FMA.
	* config/aarch64/aarch64.cc
	(aarch64_override_options_internal): Set
	param_fully_pipelined_fma according to tuning option.
	* config/aarch64/tuning_models/ampere1.h: Add
	AARCH64_EXTRA_TUNE_FULLY_PIPELINED_FMA to tune_flags.
	* config/aarch64/tuning_models/ampere1a.h: Likewise.
	* config/aarch64/tuning_models/ampere1b.h: Likewise.