erasure-code: isa uses per chunk alignment constraints

Copy code from the jerasure plugin to enforce alignment constraints per
chunk instead of using the total object size. It is simpler and reduces
the size of the chunks. See
https://github.com/ceph/ceph/commit/c7daaaf5e63d0bd1d444385e62611fe276f6ce29
for more information.

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
erasure-code: isa plugin alignment must not be architecture dependent

Replace the EC_ISA_VECTOR_OP_WORDSIZE value (which is architecture /
compiler dependent) with LARGEST_VECTOR_WORDSIZE. Otherwise it would be
possible for two isa binaries to come up with different chunk sizes
because they were compiled in different environment. This in turn will
lead to OSDs disagreeing on the chunk size.

http://tracker.ceph.com/issues/9536 Fixes: #9536

Signed-off-by: Loic Dachary <loic-201408@dachary.org>