Convert to BuildArch: noarch
    
    Firmware packages do not include binaries that can be executed from
    within the installed operating system, and so are intrinsically noarch
    packages.
    
    Convert to using "BuildArch: noarch" to match the behaviour of other
    firmware packages such as edk2, seabios, openbios, sgabios, and SLOF.
    
    This change makes the arm-trusted-firmware packages available on
    non-AArch64 machines, which is useful since it is possible to create
    an AArch64 VM on a non-AArch64 host using e.g. qemu-system-aarch64.
    
    As requested by the package maintainer, retain "ExclusiveArch:
    aarch64" and make cross-compilation a non-default option dependent
    upon building with "--with cross", so that only native compilation
    will be used within the Fedora build system.
    
    Signed-off-by: Michael Brown <mbrown@fensystems.co.uk>