RT_USING_STDC_ATOMIC is a user selected option on current config system.
While the RT_USING_HW_ATOMIC is a forced option selected by Kconfig
under libcpu. And the RT_USING_STDC_ATOMIC will be meaningless if we set
RT_USING_HW_ATOMIC to first priority if the arch has hw-atomic.
Changes:
- set RT_USING_STDC_ATOMIC to first priority on rttypes.h
Signed-off-by: Shell <smokewood@qq.com>