lib/crypto: powerpc/md5: Migrate optimized code into library

Instead of exposing the powerpc-optimized MD5 code via powerpc-specific
crypto_shash algorithms, instead just implement the md5_blocks() library
function.  This is much simpler, it makes the MD5 library functions be
powerpc-optimized, and it fixes the longstanding issue where the
powerpc-optimized MD5 code was disabled by default.  MD5 still remains
available through crypto_shash, but individual architectures no longer
need to handle it.

Link: https://lore.kernel.org/r/20250805222855.10362-5-ebiggers@kernel.org


Signed-off-by: Eric Biggers <ebiggers@kernel.org>