arch: custom cpu_idle and cpu_atomic harmonization

custom arch_cpu_idle and arch_cpu_atomic_idle implementation was done
differently on different architectures. riscv implemented those as weak
symbols, xtensa used a kconfig and all other architectures did not
really care, but this was a global kconfig that should apply to all
architectures.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>