Make logcount act like CL on negative arg

Make logcount act like CL on negative arg

Behaving like Common Lisp is less likely to lead to surprises,
as it yields the same answers on 32- vs 64-bit machines.
* doc/lispref/numbers.texi (Bitwise Operations):
Document behavior on negative integers.
* src/data.c (Flogcount):
Behave like Common Lisp for negative arguments.
* test/src/data-tests.el (data-tests-popcnt)
(data-tests-logcount): Test negative args too.