Add kernel-cross-headers subpackage
    
    The kernel-headers package it primarily used in Fedora to build the
    toolchain components.  This works fine for native builds, but we do
    not have cross-architecture headers available.  That means that the
    cross compiler toolchains we have can only be built without headers
    and therefore are only useful for building the kernel.  Given that
    Fedora does not allow cross-compiling of it's packages, that hasn't
    been a problem to date.
    
    However, the toolchains might be used by application developers to
    build their own software.  Providing the ability to build cross
    toolchains that offer glibc as well may benefit these users.  This
    provides a kernel-cross-headers package that will at least allow
    this possibility.