Rollup merge of #149539 - quaternic:gather-scatter-bits, r=Mark-Simulacrum

Additional test for uN::{gather,scatter}_bits

Feature gate: #![feature(uint_gather_scatter_bits)]
Tracking issue: rust-lang/rust#149069
Accepted ACP: rust-lang/libs-team#695 (comment)

Adds an additional runtime test for `uN::gather_bits` and `uN::scatter_bits` in coretests. They are each other's inverses in a sense, so a shared test can test both with relative ease.

I plan to follow up with optimized implementations for these functions.

Author: matthiaskrgr