Arch Linux ARM kernels are built with mainstream source from kernel.org, and includes Arch Linux and community-supported, device-specific patches. You can make your own kernel on Arch Linux ARM to add or remove features or build modules that might not exist in the default kernel.
When making kernels, it's best to build it as a package so that it can be easily installed, upgraded, or removed. Our kernel packages for different architectures can be found on GitHub. The "linux" package is for the Marvell Kirkwood platforms we support, with kernels for our other supported architectures prefixed with "kernel26-" and "linux-" in that directory.
For detailed instructions on building kernels, see the ArchWiki.