PATCH:[darwin] fix load of a misaligned double word

[David Edelsohn]

>>>>> Richard Henderson writes:

Richard> Q slipped in from s390?

	The rs6000 backend has a 'Q' constraint used by string
instructions.  No one had bothered to add EXTRA_MEMORY_CONSTRAINT with
that letter when the macro was introduced.  Reload had not run into this
problem with the 'Q' constraint, but it should not hurt to add it as well.

David