tree-wide: don't play games with alignment around file_handle

tree-wide: don't play games with alignment around file_handle

The payload of a file_handle structure is not 64bit aligned. So far used
_alignas_() to align it to 64bit as a whole, which by accident has the
side-effect that the payload ends up being aligned to 64bit too, but
this is ugly, because it's really just an accident...

Let's do this properly, and just use proper unaligned 64bit reads to
access the field, and do not assume aligning the structure as a whole
also aligns the payload part of it.

Follow-up for: fd51a7d8b5b1c710c3b59e877bc35b2739dcc350