gnu: ffmpeg: Build with rav1e on more systems.

* gnu/packages/video.scm (ffmpeg)[inputs]: Add rav1e on all systems
where it is supported.
[arguments]: Add flag to use rav1e when it is added as an input.

Change-Id: I53cc11cd406b165dc366ebe547ae6574ea9f58a3

gnu/packages/video.scm

@@ -1681,9 +1681,7 @@ operate properly.")
     (build-system gnu-build-system)
     (inputs
      (append
-      ;; XXX: rav1e depends on rust, which currently only works on x86_64.
-      ;; See also the related configure flag when changing this.
-      (if (target-x86-64?) (list rav1e) '())
+      (if (supported-package? rav1e) (list rav1e) '())
       (list dav1d
             fontconfig
             freetype
@@ -1796,7 +1794,7 @@ operate properly.")
          "--enable-libmp3lame"
          "--enable-libopus"
          "--enable-libpulse"
-         #$@(if (target-x86-64?)
+         #$@(if (this-package-input "rav1e")
                 '("--enable-librav1e")
                 '())
          "--enable-libsoxr"