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

* cipher/crc-ppc.c (CRC_VEC_U64_LOAD_BE): Move implementation to...
(asm_vec_u64_load_be): ...here; Add "r0" to clobber list for load
instruction when offset is not zero; Add zero offset path.

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>