Store the original stack pointer (on entry to the syscall dispatcher)
in syscall_frame; the stack pointer itself is incremented by
"pop {r0-r3}" right before calling the syscall itself.

This fixes unwinding from functions set up by syscalls, like
KiUserExceptionDispatcher.
Signed-off-by: Martin Storsjö <martin@martin.st>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>