libm: implement accelerated computation of `(x << e) % y`

[quaternic]

Builds on rust-lang/compiler-builtins#1011, both were split from rust-lang/compiler-builtins#1002

The computation is implemented in linear_mul_reduction.

[quaternic]

I've now rebased and applied the previous feedback.

@tgross35

Could you add a module-level doc comment with some of the common names used here? E.g. R, RR (if that's not R*R) r, m, q, xq. I'm unfortunately a bit lost :) (but I don't need to understand it in detail)

To make the module easier to follow, I moved the public interface, fn linear_mul_reduction, to the start of the file and added more explanatory comments. That should be understandable on its own. The implementation is essentially unchanged from before, other than one minor tweak.

[quaternic]

I've gone through and rewritten the comments for the implementation of Reducer, which should actually be understandable now. (At least this time I didn't get too much of a headache from it.)

[rustbot]

:warning: Warning :warning:

• The following commits have merge commits (commits with multiple parents) in your changes. We have a no merge policy so these commits will need to be removed for this pull request to be merged.

  • 43fc861484dc9dda855f415c41dea70043fb196e

  You can start a rebase with the following commands:

  $ # rebase
  $ git pull --rebase https://github.com/rust-lang/compiler-builtins.git master
  $ git push --force-with-lease