xtensa: userspace: align stack before calling syscall handler

We stuff the 7th argument into stack by moving the stack pointer
before calling syscall handler. The Xtensa ABI says stack must
be 16-byte aligned. So instead of moving stack pointer 4 bytes,
we move 16 bytes (assuming stack has been aligned so far).

Signed-off-by: Daniel Leung <daniel.leung@intel.com>