maint: prefer stdbit.h to count-leading-zeros.h

<stdbit.h> is in C23 and should be more portable in the long run,
now that Gnulib supports it.
* bootstrap.conf (gnulib_modules): Remove count-leading-zeros.
Add stdc_leading_zeros.
* gl/lib/randperm.c, src/ioblksize.h:
Include stdbit.h instead of count-leading-zeros.h.
* gl/lib/randperm.c (floor_lg): Remove; no longer needed.
(randperm_bound): Use stdc_bit_width instead of floor_lg;
* gl/modules/randperm (Depends-on): Remove count-leading-zeros.
Add stdc_bit_width.
* src/ioblksize.h (io_blksize): Use stdc_leading_zeros_ull
instead of count_leading_zeros_ll.

bootstrap.conf

@@ -64,7 +64,6 @@ gnulib_modules="
   config-h
   configmake
   copy-file-range
-  count-leading-zeros
   crypto/md5
   crypto/sha1
   crypto/sha256
@@ -247,6 +246,7 @@ gnulib_modules="
   stat-size
   stat-time
   stdbool
+  stdc_leading_zeros
   stdckdint
   stdlib-safer
   stpcpy