fix: correct padding with negative values in strftime

[ChrisDryden]

This is a proposed fix for #485 where the padding values when using a space as the padding byte adds the '-' sign before the spaces. This approach tried to separate the logic for adding the padding from the happy path since there has been a bunch of optimization work done to make this faster and its a rare case to have negative numbers.

The overarching purpose of this fix is to use it in uutils coreutils: uutils/coreutils#10242