Remove the :i486 feature.

code/misc.lisp::
* Remove :i486 from *features*

code/multi-proc.lisp::
* Change :i486 to :x86 on the off chance that multi-proc will be
  ported to other archs.

compiler/x86/cell.lisp::
compiler/x86/macros.lisp::
compiler/x86/memory.lisp::
* Remove guard on backend-featurep :i486 since we got rid of :i486 and
  have it always enabled now.