rtos/os-decls.h: more common decls
rtos/os-decls.h: more common decls

- Critical_section, Uncritical_section
- namespace scheduler
- namespace this_thread, thread
- Thread
- Systick_clock

micro-os-plus/cmsis-plus,micro-os-plus/cmsis-plus,micro-os-plus/cmsis-plus,micro-os-plus/cmsis-plus