[libc] implement stdc_leading_zeros_u* for stdbit.h (#79669)

- stdbit.stdc_leading_zeros_uc
- stdbit.stdc_leading_zeros_us
- stdbit.stdc_leading_zeros_ui
- stdbit.stdc_leading_zeros_ul
- stdbit.stdc_leading_zeros_ull

Test via:
$ ninja libc-stdbit-tests libc_include_tests