crypto: powerpc - Factor out the core CRC vpmsum algorithm

The core nuts and bolts of the crc32c vpmsum algorithm will
also work for a number of other CRC algorithms with different
polynomials. Factor out the function into a new asm file.

To handle multiple users of the function, a user simply
provides constants, defines the name of their CRC function,
and then #includes the core algorithm file.

Cc: Anton Blanchard <anton@samba.org>
Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>