Replace Ctxt.Mode with a method, Ctxt.RegWidth,
which is calculated directly off the arch info.

I believe that Prog.Mode can also be removed; future CL.

This is a step towards obj.Link immutability.

Passes toolstash-check -all.

Updates #15756

Change-Id: Ifd7f8f6ed0a2fdc032d1dd306fcd695a14aa5bc5
Reviewed-on: https://go-review.googlesource.com/38446
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>