* [add] the function realization of signal for posix.
* [update] the posix support for armclang.
* [add] the new macro "RT_USING_POSIX_TIMER".
* [modify] select "RT_USING_SOFT_TIMER" when use posix'timer.
* [bug] optimize the logic for the "time_xxx" functions.
* [modify] use "RT_USING_POSIX_TIMER" to protect the macro definition.
* [modify] error code when except happened.
* [delete] the "environ" is useless at this time.
* [posix] POSIX standard implementation for PSE51
- add some posix's interfaces that we haven't before.
- these PR have passed the interface definition test across gcc platfrom;
- have tested base on qemu-a9 and stm32h750-art-pi.
* [newlib] only enable POSIX.1-1990
* update projects
guozhanxin
xiangxistu
Man, Jianting (Meco)