rijndael-ppc: fix bad register used for vector load/store assembly

* cipher/rijndael-ppc.c (vec_aligned_ld, vec_load_be, vec_aligned_st)
(vec_store_be): Add "r0" to clobber list for load/store instructions.

Register r0 must not be used for RA input for vector load/store
instructions as r0 is not read as register but as value '0'.

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>