sh: Fix optimized __copy_user() movca.l usage.

movca.l is restricted to SH-4 and up only, though compilers that
are unable to support ISA tuning (especially older versions of
binutils) will happily compile in the bogus opcode on older parts.

Conditionalize it to fix SH-3 regressions noted by Kristoffer.

Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>