redhat: Add arm64-16k kernel flavor scaffold for 16K page-size'd AArch64

This modifies the kernel build to add a kernel-16k flavor for AArch64, which is particularly useful for ARM platforms optimized to prefer operating with a 16K page size, such as the Apple Silicon platforms.

This change is modeled after the changes done to introduce kernel-64k for RHEL, though the flavor is not going to be built at this time.