gnu: svt-hevc: Mark as supported on x86_64-linux only.

This package fails to build on i686-linux: it makes use of SSE2 and
related intrinsics not supported on i686.

* gnu/packages/video.scm (svt-hevc)[supported-systems]: Remove
"i686-linux".
* gnu/packages/gstreamer.scm (gst-plugins-bad)[inputs]: Adjust condition
accordingly.

gnu/packages/gstreamer.scm

@@ -889,9 +889,9 @@ model to base your own plug-in on, here it is.")
        ("soundtouch" ,soundtouch)
        ("spandsp" ,spandsp)
        ("srt" ,srt)
-       ,@(if (target-x86?)
-           `(("svthevcenc" ,svt-hevc))
-           '())
+       ,@(if (target-x86-64?)
+             `(("svthevcenc" ,svt-hevc))
+             '())
        ("tinyalsa" ,tinyalsa)
        ("transcode" ,transcode)
        ("usrsctp" ,usrsctp)

gnu/packages/video.scm

@@ -335,7 +335,7 @@ the SVT-HEVC encoder, it is possible to spread video encoding processing across
 multiple Intel's Xeon processors to achieve a real advantage of processing
 efficiency.")
     (home-page "https://01.org/svt")
-    (supported-systems '("x86_64-linux" "i686-linux"))
+    (supported-systems '("x86_64-linux"))
     (license (license:non-copyleft "file:///LICENSE.md"))))
 
 (define-public mediasdk