[libc++][chrono] Fixes format output of negative values. (#89408)

When trying to express a time before the epoch (e.g. "one nanosecond
before 00:01:40 on 1900-01-01")
the date would be shown as:

  1900-01-01 00:01:39.-00000001

After this patch, that time would be correctly shown as:

  1900-01-01 00:01:39.999999999