Fix offsets of source tracking hooks in the disassembler.

The disassembler by default annotates code compiled under a sufficiently
high DEBUG optimization quality with source code fragments. This was
mostly broken: these annotations were being printed in the wrong place
(many lines too far into the output) or not at all.

The reason for this behaviour was that ADD-SOURCE-TRACKING-HOOKS
calculated the annotation's offsets relative to the start of the
instructions in the code component and passed them unchanged to the
hooks used to print the annotations, which expect them to be relative
to the start of the segment.

Fix this by adding code in ADD-SOURCE-TRACKING-HOOKS to convert the
offsets appropriately.

This bug seems to have been inherited already from CMUCL and it only
attracted attention when a change to the handling of prefix instructions
added a new failure mode to the existing ones, namely source annotations
breaking up disassembly lines inmidst the instruction bytes. See
lp#1249205 for details.

Fixes lp#1249205.
Teach monitor's 'dump' command about 8-byte words.
Placate Darwin C compiler with an explicit function prototype.
Remove ASSIGN-SETF-MACRO's secondary test for fboundness of SETF fun.

The new code is behaviorally different in a subtle way.  The old didn't
didn't warn if a setf function was defined earlier in the same file
unless in eval-when :compile-toplevel. Whereas if (SETF F) had come
from a previously loaded file, you did get a style-warning upon
definition of the macro. Now either way style-warns.

There is no change in the signaling of a full warning if the compiler
has *assumed* a setf function to exist when the macroish setter is
defined, because that suggests something was compiled wrong.

This is almost but not entirely like the patch I proposed on sbcl-devel.
The unanticipated problems were mainly during xc because :where-from
changes what it reports for a function name that has no binding.
This might possibly warn about a few things in xc that it didn't before.
CURRENT-THREAD-OFFSET-SAP, take 2, now with less magic.