aarch64: Set Armv9-A generic L1 cache line size to 64 bytes

I'd like to use a value of 64 bytes for the L1 cache size for Armv9-A
generic tuning.
As described in g:9a99559a this value is used
to set the std::hardware_destructive_interference_size value which we want to
be not overly large when running concurrent applications on large core-count
systems.

The generic value for Armv8-A systems and the port baseline is 256 bytes
because that's what the A64FX CPU has, as set de-facto in
aarch64_override_options_internal.

But for Armv9-A CPUs as far as I know there isn't anything larger
than 64 bytes, so we should be able to use the smaller value here and reduce
the size of concurrent structs that use
std::hardware_destructive_interference_size to pad their fields.

Bootstrapped and tested on aarch64-none-linux-gnu.

Signed-off-by: Kyrylo Tkachov <ktkachov@nvidia.com>

	* config/aarch64/tuning_models/generic_armv9_a.h
	(generic_armv9a_prefetch_tune): Define.
	(generic_armv9_a_tunings): Use the above.