Fix the build on OS X 10.8.0

It seems our exception handler can be called before it's fully set up.
Handle that case without potentially leaking too many ports.

Reported by Gabriel Dos Reis on sbcl-devel.
Substitute constants with modular equivalents more safely

* Modular arithmetic sometimes lets us narrow constants down,
  especially with signed arithmetic. We now update the receiving
  LVAR's type conservatively when there are multiple uses; otherwise,
  conflicting type information results in spurious dead code
  elimination.

* Test case by Eric Marsden.

* Reported by Eric Marsden on sbcl-devel (2013-04-18).
Trivial code cleanups

 Declare a variable as ignored, and descriptors are 64 bit on
 x86-64. The latter was brought to my attention by Douglas Katzman.
sort NEWS into enhancement/bug fix/optimization order
fix formatting of most recent "changes" line in NEWS
1.1.7: will be tagged as "sbcl-1.1.7"
Merge tag 'refs/tags/sbcl-1.1.7' into doc-ja

changes in sbcl-1.1.7 relative to sbcl-1.1.6:
  * enhancement: TRACE :PRINT-ALL handles multiple-valued forms.
    (lp#457053)
  * bug fix: :allocation slot option works for condition slots
    (lp#1049404)
  * bug fix: redefining conditions does not lead to multiple evaluations of
    hairy slot initfunctions anymore (lp#1164969)
  * bug fix: CLASS-DIRECT-DEFAULT-INITARGS now works for condition classes
    (lp#1164970)
  * bug fix: function constants now work as initforms and default initarg
    values of conditions (lp#539517)
  * bug fix: svref/(setf svref) on symbol macros don't crash the compiler
    anymore. (Minimal test case provided by James M. Lawrence on sbcl-devel)
  * bug fix: no more bogus ## references when pretty printing backquoted
    forms with non-trivial structure sharing. (lp#1161218)
  * bug fix: Don't signal an error on (setf (documentation nil 'function)
    "doc").  Signal a style-warning instead when trying to set documentation
    of NIL for all other documentation types.  (Reported by Zach
    Beane; regression since 2e52fa05.)
  * bug fix: modular arithmetic involving large constants and conditionals
    should no longer result in spurious dead code elimination.  (Reported by
    Eric Marsden).
  * bug fix: our mach exception handler can seemingly called very early in
    the program execution process on OS X 10.8.0. Try and handle that case
    robustly, without potentially leaking mach ports too much.
  * optimization: LOGBITP and LOGTEST optimizations from x86 ported to x86_64.
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