kernel: make MULTITHREADING promptless if single-thread not supported

If single thread builds are not supported by the
architecture, the MULTITHREADING option should be
prompt-less to block any modifications to it. We
also introduce an explicit ARCH-level Kconfig that
reflects whether the ARCH is capable of single-thread
Zephyr builds.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>