fix: Unstable library feature .is_multiple_of

[shrehanrajsingh]

use of .is_multiple_of, which is an unstable library feature Implemented custom is_multiple_of with bound check Also added a dockerfile and ccc-docker.sh that runs a C file in a container since I couldn't find a way to run this compiler on MacOS.

This pull request introduces a new utility function for checking multiples, refactors codebase-wide usages of this logic to use the new utility, and adds Docker-based tooling for building and running the compiler in a consistent environment. The most important changes are grouped below by theme.

New Utility Function and Refactoring:

• Added a generic is_multiple_of function in src/common/is_multiple_of.rs and exposed it via src/common/mod.rs for use throughout the codebase. [1] [2]
• Replaced direct calls to .is_multiple_of() and manual modulus checks with the new crate::common::is_multiple_of::is_multiple_of function in ARM, x86, and RISC-V backend files, improving code consistency and portability. [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

Build and Tooling Improvements:

• Added a Dockerfile to build the compiler and its dependencies in a reproducible environment, supporting cross-compilation for multiple architectures.
• Introduced ccc-docker.sh, a script for compiling and running C files using the compiler inside Docker, simplifying usage and ensuring consistent builds across systems.

Minor Logic Adjustments:

• Replaced .is_multiple_of with custom is_multiple_of routine because Rust refused to compile code containing .is_multiple_of labelling it "Unstable library feature"

These changes collectively improve code maintainability, portability, and developer experience.