When running on x86, and riscv never enter the kernel for cache maintainance,
but use the dummy implementation of the generic base library instead.
On ARMv8 it is not necessary to enter privileged mode for cache cleaning, and
unification of instruction/data cache, but only for invalidating cache lines
at all levels, which is necessary for the use cases, where this function it
needed (coherency of DMA memory).
Fixgenodelabs/genode#4339
