Provide an implementation of `bn_mul_mont` that works on all targets that
don't have an assembly language implementation.

Expand `prefixed_export!` to support prefixing functions defined in Rust.
Function definitions don't end with a semicolon so move the semicolon
insertion from `prefixed_item!` to its callers.

Unify the codepaths in `bigint` so that `bn_mul_mont` is always used.

Take MIPSEL out of the GitHub Actions configuration because it fails to
link, and because it makes the build matrix too large.