Merge pull request #150 from grissiom/simulator-linux

Simulator linux
Merge pull request #151 from grissiom/finsh

finsh: use alias in the variable definition
add __rt_ffs prototype in rtthread.h

Note that we define __rt_ffs as __builtin_ffs in GCC. __builtin_ffs will
be inlined automatically.

Build passed with GCC/Keil in bsp/stm32f10x(RT_USING_CPU_FFS enabled).