Implement `checked_mod` for `i64`

Start at `OpEmitter::checked_mod` and add `OpEmmitter::checked_mod_i64` that calls the new intrinsic (add it) at `codegen/masm/intrinsics/i64.masm`.

The issue points to OpEmitter::checked_mod and codegen/masm/intrinsics/i64.masm. Start by locating the OpEmitter struct and its checked_mod method to understand the pattern. Then, examine the existing i64 intrinsics in the specified file to see how new ones are added. Implement checked_mod_i64 by following the same conventions, ensuring it calls a new intrinsic. Verify by building the compiler and running any relevant tests for i64 operations.

Written by the indexing model from the issue text.