aarch64: modify Ampere CPU tunings on reassociation/FMA

1. Allow reassociation on FP additions.

2. Avoid generating loop-dependant FMA chains. Added a tuning
option 'AARCH64_EXTRA_TUNE_AVOID_CROSS_LOOP_FMA' for this.

gcc/ChangeLog:

	* config/aarch64/aarch64-tuning-flags.def
	(AARCH64_EXTRA_TUNING_OPTION): New tuning option to avoid
	cross-loop FMA.
	* config/aarch64/aarch64.cc
	(aarch64_override_options_internal): Set
	param_avoid_fma_max_bits according to tuning option.
	* config/aarch64/tuning_models/ampere1.h (ampere1_tunings):
	Modify tunings related with FMA.
	* config/aarch64/tuning_models/ampere1a.h (ampere1a_tunings):
	Likewise.
	* config/aarch64/tuning_models/ampere1b.h (ampere1b_tunings):
	Likewise.