x86 sap fixes

x86 sap fixes

The sap-ref-with-offset stuff was wrong in that the displacement was
multiplied by a notional element size, rather than being uniformly
treated as a number of bytes.  Mostly this codepath wasn't exposed at
all (other than with sap-ref-8, which worked by "accident"), but
attempts to implement UTF-16, which requires (sap-ref-16 sap (+ offset
2)), showed up the problem.