[POWERPC] Use mtocrf instruction in asm when CONFIG_POWER4_ONLY=y

mtocrf is a faster single-field mtcrf (move to condition register
fields) instruction available in POWER4 and later processors.  It can
make quite a difference in performance on some implementations, so use
it for CONFIG_POWER4_ONLY builds.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>