Made cpu-core-count sensitive to environment variable ACL2_CORE_COUNT.

Quoting :doc note-7-0:

  (ACL2(p) and ACL2(hp) only) The function [Cpu-core-count] is now
  sensitive to environment variable ACL2_CORE_COUNT. See
  [cpu-core-count]. Thanks to Jared Davis for requesting this
  enhancement.

Tweaked :doc case, thanks to feedback from Ben Selfridge.