cmd/asm: add a couple of operand parses discovered by end-to-end test

Missing cases for JMP $4 and foo+4(SB):AX. Both are odd but 8a accepts them
and they seem valid.

Change-Id: Ic739f626fcc79ace1eaf646c5dfdd96da59df165
Reviewed-on: https://go-review.googlesource.com/5693


Reviewed-by: Russ Cox <rsc@golang.org>