target-alpha: fix cmpbge instruction

The cmpbge instruction should compare all 8 bytes of one 64-bit value with
another.  However, we were looping with a < 7 condition which was skipping
the top byte.  So if we were doing a compare where the top byte was
important, we could get the wrong result (this notably breaks the strlen()
function with certain sized strings).

(Vince Weaver)

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5667 c046a42c-6fe2-441c-8c8c-71466251a162
