SH: fix start_thread and user_stack_pointer macros

Fix macros start_thread and user_stack_pointer.
When these macros aren't called with a variable named regs as second
argument, this will result in a build failure.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>