Reduce size of integer product in timefns

Reduce size of integer product in timefns

* src/timefns.c (emacs_gcd): New static function.
(ticks_hz_hz_ticks): Use it to reduce the size of the integer
product in the common case when converting from ns to ps.  For
that, we need to multiply t.ticks only by 10³, not multiply by
10¹² and then divide by 10⁹.  This avoids the need to use bignums
in a significant number of cases.

(cherry picked from commit 6ef052d9b23aef3f34d19e5d93d97884215d1465)