factor: switch to stdc_leading_zeros etc

* bootstrap.conf (gnulib_modules): Add stdc_trailing_zeros.
* cfg.mk (_gl_TS_unmarked_extern_vars): Remove factor_clz_tab,
as it’s no longer present.
* src/factor.c: Include stdbit.h.
(__clz_tab, factor_clz_tab): Remove.
(ASSERT, UHWtype, __GMP_DECLSPEC): Use simpler way to pacify
-Wunused-macros.
(count_leading_zeros, count_trailing_zeros):
Remove.  All uses replaced by stdc_leading_zeros, stdc_trailing_zeros.
(factor_using_division, prime2_p): Add a couple of ‘assume’s
so that GCC knows the stdc_* calls are nonzero and can
optimize accordingly.

bootstrap.conf

@@ -247,6 +247,7 @@ gnulib_modules="
   stat-time
   stdbool
   stdc_leading_zeros
+  stdc_trailing_zeros
   stdckdint
   stdlib-safer
   stpcpy