sh: Prevent leaking of CONFIG_SUPERH32 to userspace in asm/unistd.h.

CONFIG_SUPERH32 is currently trickling into userspace unistd.h. Attached
patch uses __SH5__ define in userspace.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>