Rollup merge of #142114 - GuillaumeGomez:u128-const, r=Urgau

Compute number of digits instead of relying on constant value for u128 display code

As discussed in https://github.com/rust-lang/rust/pull/142098/files#r2132084991, the code should reuse the same logic as the rest of file instead of using a constant value.

r? `@tamird`

Author: GuillaumeGomez