arch: arm: push ssf to thread privileged stack to complete stack frame

All system call handlers have the same prototype:
u32_t _handler_APINAME(u32_t arg1, u32_t arg2, u32_t arg3,
		       u32_t arg4, u32_t arg5, u32_t arg6, void *ssf);

This commit pushes the seventh argument named ssf to thread's privileged
stack to avoid mis-understanding the stack layout with compiler.

Fixes: #29386.

Signed-off-by: Wentong Wu <wentong.wu@intel.com>