[POWERPC] Make altivec code in swsusp_32.S depend on CONFIG_ALTIVEC

This makes the altivec code in swsusp_32.S depend on CONFIG_ALTIVEC to
avoid build failures for systems that don't have altivec. I'm not sure
whether the code will actually work for other systems, but it was merged
for just ppc32 rather than powermac a very long time ago.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>