aarch64: Assume zero gather/scatter set-up cost for -mtune=generic

generic_vector_cost is not currently used by any SVE target
by default; it has to be specifically selected by -mtune=generic.
Its SVE costing has historically been somewhat idealised, since
it predated any actual SVE cores.  This seems like a useful
tradition to continue, at least for testing purposes.

The ideal case is that gathers and scatters do not induce a specific
one-off overhead.  This patch therefore sets the gather/scatter init
costs to zero.

This patch is necessary to switch -mtune=generic over to the
"new" vector costs.

gcc/
	* config/aarch64/tuning_models/generic.h (generic_sve_vector_cost):
	Set gather_load_x32_init_cost and gather_load_x64_init_cost to 0.