SPARC: disable VIS for Niagara CPU

The Niagara/T1 supports only a subset of VIS, and even this is very slow.

Patch by Michael Kostylev <michael kostylev gmail>

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@23045 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -2018,6 +2018,10 @@ elif enabled x86; then
 elif enabled sparc; then
 
     case $cpu in
+        niagara)
+            cpuflags="-mcpu=$cpu"
+            disable vis
+        ;;
         sparc64)
             cpuflags="-mcpu=v9"
         ;;