buffer: improve write(U)Int functions

This improves the performance of some write functions by around 5-15%.

PR-URL: https://github.com/nodejs/node/pull/19289


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>