Extract stack zero filling into new function.

This patch is based on the off road conversation with @mattn.
In some targets, it's possible getting more speed by replacing zero filling loop to memset().
But there is some risks. The one is compatibility issue. And memset() is not always fast.

So I suggest extract zero filling into new function "stack_clear()".
It make the porting developer enable to choice replacing memset() or not, as his own risks.