[nrf fromtree] arch: riscv: no_multithreading: CPU idle after return from main

So far, forever busy loop with interrupts disabled was executed when
CONFIG_MULTITHREADING=n and main() returned. Change it to forever
k_cpu_idle() which is better from power consumption perspective.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>
(cherry picked from commit c7d12998426df8f57b3e9188fae1877bc9a889aa)

Author: nordic-krch

arch/riscv/core/thread.c

@@ -295,6 +295,7 @@ FUNC_NORETURN void z_riscv_switch_to_main_no_multithreading(k_thread_entry_t mai
 	/* infinite loop */
 	irq_lock();
 	while (true) {
+		k_cpu_idle();
 	}
 
 	CODE_UNREACHABLE; /* LCOV_EXCL_LINE */