cpu: fix offset16 type, update c-calls in bpf.cpu

Correct the type of the offset16 field to HI, and simplify memory
accesses which use it. Also update c-calls in semantics for a
few instructions.

cpu/ChangeLog:

2020-06-25 David Faust  <david.faust@oracle.com>

	* bpf.cpu (f-offset16): Change type from INT to HI.
	(dxli): Simplify memory access.
	(dxsi): Likewise.
	(define-endian-insn): Update c-call in semantics.
	(dlabs) Likewise.
	(dlind) Likewise.