Remove :PENTIUM from *FEATURES*

code/misc.lisp::
* Don't register :PENTIUM feature.

code/time.lisp::
* Replace :pentium with :x86; we assume all x86's have the rdtsc
  instruction.

compiler/x86/float.lisp::
compiler/x86/system.lisp::
* Remove the vop guard on :pentium; assume we're always running on a
  pentium or better.