Revert "kernel/process: remove declaration of arch_idle which i commented out"

This reverts commit bebb718a75764ab3e6fe02c60a6112e61721ee2a.
exynos: uncomment this for now

Change-Id: Iaf6c8938fb73d25f5c6b20a4a2b3630175be41a5
Signed-off-by: zwliew <zhaoweiliew@gmail.com>