[php-src] Issue #14675: Indicate expected improvement in BcMath's performance

Issue: https://github.com/php/php-src/issues/14675 Author: FleetingCloud ### Description Hi, first of all, thanks a lot for all the work done on BcMath especially Saki Takamachi, nielsdos and Gina. Currently NEWS file only include following about it: > Improve performance. (Saki Takamachi, nielsdos) I think it should also tell by *how much* . It's because a person will likely think that improvement is modest if they just read above news entry. By including some benchmark, it will encourage people heavily using BcMath to update their PHP version sooner. It will also help improve perception of "BCMath is slow" which it is not now. Also spread more awareness. Again thank a lot. Cc @SakiTakamachi