caused memory destruction when kernel stack grow over 1 page. new
code use P3 address for kernel stack. but for the sake of debug, P1
kernel stack mode remain.
sh3/sh3/exception_vector.S and sh3/sh3/locore_subr.S. exception
vectors are installed by sh_cpu_init(). machine/locore.S contains
kernel entry, interrupt handler, and some MD code.