amd64: tweak the read_frequently section

1. align to 128 bytes to avoid possible waste from the preceeding section
2. sort entries by alignment SORT_BY_ALIGNMENT, plugging the holes (most

entries are one byte in size, but they got interleaved with bigger ones)

Interestingly I was looking for a feature of the sort earlier and failed
to find it. It turns out the script was already utilizing sorting in other
places, so shame on me.

Thanks for Travis Geiselbrecht for pointing me at the feature.