When the 64bit atomics simulation is in use, we can't necessarily
guarantee the correct alignment of the atomics due to lack of compiler
support for doing so- that's fine from a safety perspective, because
everything is protected by a lock, but we asserted the alignment in
all cases.  Weaken them.  Per complaint from Alvaro Herrera.

My #ifdefery for PG_HAVE_8BYTE_SINGLE_COPY_ATOMICITY wasn't
sufficient. Fix that.  Per complaint from Alexander Korotkov.