[disco/master] nvidia build fixes

[Andy Whitcroft]

Following this email are a number of packaging fixes for the in kernel
Nvidia build.  These fix both the primary kernel build and also some issues
in the to be uploaded LRM packaging which syncs these exact same tools.

Proposing for application to disco.

-apw

    UBUNTU: [Packaging] dkms-build--nvidia-N -- remove GCC versions
    UBUNTU: [Packaging] fix-filenames -- handle exact string removal
    UBUNTU: [Packaging] dkms-build--nvidia-* -- convert to generic -N form
    UBUNTU: [Packaging] dkms-build -- add support for unversioned overrides
    UBUNTU: [Packaging] dkms-build--nvidia* -- check gcc version against primary build
    UBUNTU: [Packaging] dkms-build -- remove retpoline data from final binary packages
    UBUNTU: [Packaging] dkms-build -- elide partial Built-Using information
    UBUNTU: [Packaging] dkms-build: do not redownload files on subsequent passes
    UBUNTU: [Packaging] dkms-build -- support building against packages in PPAs