sh: Make sure AT_SYSINFO_EHDR is exposed to userspace in asm/auxvec.h.

Presently this is protected by a CONFIG_VSYSCALL ifdef so we don't
inadvertently trigger the creation of the gate VMA on CPUs where we don't
enable the vDSO, which is obviously not visible to userspace. Fix this up
by adding in an ifndef __KERNEL__ check at the same time.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>