The assembler barfs on large offsets. Make sure that all the
instructions that need to have their offsets in an int32
  1) check on any rule that computes offsets for such instructions
  2) change their aux fields so the check builder checks it.

The assembler also silently misassembled offsets between 1<<31
and 1<<32. Add a check in the assembler to barf on those as well.

Fixes #21655

Change-Id: Iebf24bf10f9f37b3ea819ceb7d588251c0f46d7d
Reviewed-on: https://go-review.googlesource.com/59630
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>