Some architectures require 64 bit accesses to be 64 bit
aligned, which also requires structs containing 64 bit values
to be 64 bit aligned too. This includes some 32 bit
architectures which can do 64 bit accesses, as well as 64 bit
architectures without unaligned access.

This symbol should be selected by an architecture if 64 bit
accesses are required to be 64 bit aligned in this way even
though it is not a 64 bit architecture.

See Documentation/unaligned-memory-access.txt for more
information on the topic of unaligned memory accesses.