x86: Use 64MB as nt-store threshold if no cacheinfo [BZ #30429]

If `non_temporal_threshold` is below `minimum_non_temporal_threshold`,
it almost certainly means we failed to read the systems cache info.

In this case, rather than defaulting the minimum correct value, we
should default to a value that gets at least reasonable
performance. 64MB is chosen conservatively to be at the very high
end. This should never cause non-temporal stores when, if we had read
cache info, we wouldn't have otherwise.
Reviewed-by: Florian Weimer <fweimer@redhat.com>