gnu: Restore emacs' supported systems.

gnu: Restore emacs' supported systems.

Recently, librsvg was updated to depend upon rust.  That change inadvertently
restricted the "supported" systems of emacs (among other packages) to
x86_64-linux only, since that is the only system declared in the rust
package's list of supported systems.  This unintentionally made emacs
unavailable on all other systems.

This change fixes that by removing the rust dependency from some packages.
The packages remain unchanged from the perspective of an x86_64-linux system,
but from the perspective of other systems, the packages are now supported
again (albeit without certain optional SVG features).

* gnu/packages/gtk.scm (gtk+, gtk+-2)[propagated-inputs]: If compiling for
x86_64, use gdk-pixbuf+svg as the "gdk-pixbuf" input, as usual.  Otherwise,
use gdk-pixbuf, which avoids adding librsvg (thus rust) to the closure of
inputs.  Note that both gtk+ and gtk+-2 are in the transitive closure of
inputs of emacs, so these two changes are necessary.
* gnu/packages/emacs (emacs)[inputs]: If compiling for x86_64, add librsvg to
the inputs, as usual.  Otherwise, do not add it, which avoids adding rust to
the closure of inputs.