During the review of CL 38801 it was noted that it would be nice
to have a bit more clarity on how-and-why SB addressing is handled
strangely on s390x. This additional comment should hopefully help.

In general SB is handled differently because not all instructions
have variants that use relative addressing.

Change-Id: I3379012ae3f167478c191c435939c3b876c645ed
Reviewed-on: https://go-review.googlesource.com/38952Reviewed-by: Keith Randall <khr@golang.org>