rm48x50: optimize context_switch_interrupt_to by reuse registers
rm48x50: optimize context_switch_interrupt_to by reuse registers

When saving thread registers in context_switch_interrupt_to, we don't
change them, just move them. So there is no need to always r0-r3 from
stack to the real r0-r3. So just use the intermediate registers and
eliminate 2 MOV.

nongxiaoming/rt-thread,zhaojuntao/rt-thread,gbcwbz/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,gbcwbz/rt-thread,yongli3/rt-thread,yongli3/rt-thread,gbcwbz/rt-thread,FlyLu/rt-thread,weety/rt-thread,armink/rt-thread,armink/rt-thread,weiyuliang/rt-thread,ArdaFu/rt-thread,zhaojuntao/rt-thread,hezlog/rt-thread,weiyuliang/rt-thread,weety/rt-thread,armink/rt-thread,wolfgangz2013/rt-thread,hezlog/rt-thread,igou/rt-thread,weiyuliang/rt-thread,igou/rt-thread,weety/rt-thread,weety/rt-thread,ArdaFu/rt-thread,nongxiaoming/rt-thread,ArdaFu/rt-thread,zhaojuntao/rt-thread,ArdaFu/rt-thread,AubrCool/rt-thread,weiyuliang/rt-thread,gbcwbz/rt-thread,ArdaFu/rt-thread,zhaojuntao/rt-thread,geniusgogo/rt-thread,zhaojuntao/rt-thread,FlyLu/rt-thread,AubrCool/rt-thread,nongxiaoming/rt-thread,hezlog/rt-thread,yongli3/rt-thread,RT-Thread/rt-thread,wolfgangz2013/rt-thread,zhaojuntao/rt-thread,igou/rt-thread,nongxiaoming/rt-thread,AubrCool/rt-thread,weiyuliang/rt-thread,geniusgogo/rt-thread,FlyLu/rt-thread,igou/rt-thread,yongli3/rt-thread,igou/rt-thread,hezlog/rt-thread,RT-Thread/rt-thread,armink/rt-thread,FlyLu/rt-thread,hezlog/rt-thread,hezlog/rt-thread,RT-Thread/rt-thread,gbcwbz/rt-thread,RT-Thread/rt-thread,armink/rt-thread,weety/rt-thread,yongli3/rt-thread,AubrCool/rt-thread,wolfgangz2013/rt-thread,RT-Thread/rt-thread,gbcwbz/rt-thread,wolfgangz2013/rt-thread,wolfgangz2013/rt-thread,igou/rt-thread,igou/rt-thread,gbcwbz/rt-thread,wolfgangz2013/rt-thread,yongli3/rt-thread,zhaojuntao/rt-thread,geniusgogo/rt-thread,AubrCool/rt-thread,FlyLu/rt-thread,geniusgogo/rt-thread,armink/rt-thread,geniusgogo/rt-thread,RT-Thread/rt-thread,geniusgogo/rt-thread,hezlog/rt-thread,AubrCool/rt-thread,FlyLu/rt-thread,weiyuliang/rt-thread,weety/rt-thread,weiyuliang/rt-thread,weety/rt-thread,geniusgogo/rt-thread,ArdaFu/rt-thread,ArdaFu/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,FlyLu/rt-thread,yongli3/rt-thread,armink/rt-thread,wolfgangz2013/rt-thread,AubrCool/rt-thread