vect: Add is_gather_scatter argument to misalignment hook.

vect: Add is_gather_scatter argument to misalignment hook.

This patch adds an is_gather_scatter argument to the
support_vector_misalignment hook.  All targets but riscv do not care
about alignment for gather/scatter so return true for is_gather_scatter.

gcc/ChangeLog:

	* config/aarch64/aarch64.cc (aarch64_builtin_support_vector_misalignment):
	Return true for gather/scatter.
	* config/arm/arm.cc (arm_builtin_support_vector_misalignment):
	Ditto.
	* config/epiphany/epiphany.cc (epiphany_support_vector_misalignment):
	Ditto.
	* config/gcn/gcn.cc (gcn_vectorize_support_vector_misalignment):
	Ditto.
	* config/loongarch/loongarch.cc (loongarch_builtin_support_vector_misalignment):
	Ditto.
	* config/riscv/riscv.cc (riscv_support_vector_misalignment):
	Add gather/scatter argument.
	* config/rs6000/rs6000.cc (rs6000_builtin_support_vector_misalignment):
	Return true for gather/scatter.
	* config/s390/s390.cc (s390_support_vector_misalignment):
	Ditto.
	* doc/tm.texi: Add argument.
	* target.def: Ditto.
	* targhooks.cc (default_builtin_support_vector_misalignment):
	Ditto.
	* targhooks.h (default_builtin_support_vector_misalignment):
	Ditto.
	* tree-vect-data-refs.cc (vect_supportable_dr_alignment):
	Ditto.