bigint: Provide a fallback implementation for `bn_mul_mont`.

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.

(cherry picked from commit 81f4e8d07da3f2ccc57f69a91245b41e6d764a1c)
Test: builds
Change-Id: If2cb061684ee1a0831f186c2f4cee3f02c2a236b
4 files changed