runtime: check alignment of 8-byte atomic loads and stores on 386
Currently, if we do an atomic{load,store}64 of an unaligned address on
386, we'll simply get a non-atomic load/store. This has been the
source of myriad bugs, so add alignment checks to these two
operations. These checks parallel the equivalent checks in
sync/atomic.
The alignment check is not necessary in cas64 because it uses a locked
instruction. The CPU will either execute this atomically or raise an
alignment fault (#AC)---depending on the alignment check flag---either
of which is fine.
This also fixes the two places in the runtime that trip the new
checks. One is in the runtime self-test and shouldn't have caused
real problems. The other is in tickspersecond and could, in
principle, have caused a misread of the ticks per second during
initialization.