priv-1.13: clarify that MXLEN >= SXLEN; constrain SXLEN >= UXLEN

[aswaterman]

@gfavor FYI

@jhauser-us please check my wording. It's a little obtuse because I was trying to constrain the set of legal values for SXL's WARL purposes.

[jhauser-us]

I would probably have:

If S-mode is implemented, the set of legal values that the UXL field may assume excludes those that would cause UXLEN to be greater than SXLEN.

Because: (1) Is SXLEN defined without S-mode? (2) To me it seems more natural to have UXLEN bounded by SXLEN.

[aswaterman]

Good points. PR updated with your proposed wording.

Greg approved via email, but I'll wait to merge until we have the chance to discuss this with the ARC.