arch: arc: kconfig: Define CPU_EM* syms outside Kconfig.defconfig files

Define CPU_EM4* and CPU_EM6 in arch/arc/Kconfig to make them always
available. That way, the Kconfig.defconfig definitions can skip the
type, making them incomplete if the base definition of the symbol
disappears. That makes the organization easier to understand and errors
easier to spot.

The help texts were taken from
https://gcc.gnu.org/onlinedocs/gcc/ARC-Options.html

. Help texts for
invisible symbols can be checked in the menuconfig too if you go into
show-all mode, so they're better than adding a comment.

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