factor: port to platforms

factor: port to platforms

* src/factor.c (mod2): Work even if cntd <= cnta.  The old version
of the code assumed that shifts by N had unspecified behavior
unless 0 <= N < wordsize.  Although this assumption is portable to
all known practical platforms, the C standard says these shifts
have undefined behavior and some pedantic platforms check this.
* tests/factor/create-test.sh:
* tests/local.mk (factor_tests): New test t37.