GLVND systems are well catered for on biarch configurations, however the
older 304 + 340 series will rely on ld.so.conf tricks to mask the system
libGL.so with one in the private `libdir/nvidia*/` files. This is seen
consistently across Arch, Fedora and Solus. As such we allow these fallback
paths to work so that users of snapd on glvnd enabled distros with the older
legacy drivers can still enjoy snapd + NVIDIA together.

Signed-off-by: Ikey Doherty <[email protected]>

Instead of our two hard-coded path lists, we'll form relative paths for
all optional elements in the list against common ancestors known for biarch
distributions.

This builds upon the previous work and allows us to replace or grab files
from the subdirectories, and also allows the private NVIDIA libraries to
exist either within the toplevel libdir or within a private directory, as
seen on Fedora.

The flow is changed slightly so that our first primary call for each libdir
is the only mount, and then we follow up with a simple populate call for the
libdir.

Signed-off-by: Ikey Doherty <[email protected]>

This will allow each distro to provide the native arch and the alternative
lib32 directory without us trying to guess where they go. With this change,
we'll only try to mount the alternative 32-bit biarch space if we're a
64-bit build of snapd, otherwise we'd only have the native 32-bit arch
to worry about in the first place.

Signed-off-by: Ikey Doherty <[email protected]>

Signed-off-by: Neal Gompa <[email protected]>