aarch64: Add costs for one element of a scatter store

Currently each element in a gather load is costed as a scalar_load
and each element in a scatter store is costed as a scalar_store.
The load side seems to work pretty well in practice, since many
CPU-specific costs give loads quite a high cost relative to
arithmetic operations.  However, stores usually have a cost
of just 1, which means that scatters tend to appear too cheap.

This patch adds a separate cost for one element in a scatter store.

Like with the previous patches, this one only becomes active if
a CPU selects use_new_vector_costs.  It should therefore have
a very low impact on other CPUs.

gcc/
	* config/aarch64/aarch64-protos.h
	(sve_vec_cost::scatter_store_elt_cost): New member variable.
	* config/aarch64/aarch64.c (generic_sve_vector_cost): Update
	accordingly, taking the cost from the cost of a scalar_store.
	(a64fx_sve_vector_cost): Likewise.
	(aarch64_detect_vector_stmt_subtype): Detect scatter stores.

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