kernel: kconfig: Make SCHED_IPI_SUPPORTED invisible

Toggling this symbol probably doesn't make sense, because the
architecture is already known when Kconfig runs.

SCHED_IPI_SUPPORTED is enabled through being selected by the ARC_CONNECT
(maybe that one shouldn't be configurable either) and X86_64 symbols.

Note that it's not possible to disable the symbol when it's being
selected, so trying to turn it off on e.g. X86_64 won't work either.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>