Remove dependence of stdbit on the modules count-leading-zeros,
count-trailing-zeros, and count-one-bits. stdbit is part of C23
and in the long run is more likely to be more portable, so code
should start preferring it.
* lib/stdbit.c (popcount_support): New var, if needed.
* lib/stdbit.in.h: Contain contents of count-leading-zeros.h,
count-trailing-zeros.h, and count-one-bits.h instead of including
those files. In the long run those files should be stubs that are
implemented via stdbit.
* modules/stdbit (Depends-on): Do not depend on count-leading-zeros,
count-trailing-zeros, count-one-bits.