try multi-reg dest in default_zero_call_used_regs

try multi-reg dest in default_zero_call_used_regs

When the mode of regno_reg_rtx is not hard_regno_mode_ok for the
target, try grouping the register with subsequent ones.  This enables
s16 to s31 and their hidden pairs to be zeroed with the default logic
on some arm variants.

for  gcc/ChangeLog

	* targhooks.cc (default_zero_call_used_regs): Attempt to group
	regs that the target refuses to use in their natural modes.
	(zcur_select_mode_rtx): New.
	* regs.h (struct target_regs): Add x_hard_regno_max_nregs.
	(hard_regno_max_nregs): Define.
	* reginfo.cc (init_reg_modes_target): Set hard_regno_max_nregs.