Use the stmdb instead of str to save the fn/arg

   Directly save data into stack without properly adjustment
   of stack point is dangous. For example, if a signal comes,
   kernel will put sigframe into userspace's stack, which
   will overwrite the saved data if sp is not adjusted properly.

Bug: 15195265
Change-Id: Iea0cadfd3b94d50cf40252ad52fe5950811b9192
Signed-off-by: Jiangli Yuan <a6808c@motorola.com>