configure.ac: disable enable_gallium_llvm in the !x86 case

Already implicitly handled throughout, but keep it clear and disable
gallium-llvm. This change should be a no-op.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Tobias Droste <tdroste@gmx.de>
(cherry picked from commit ce65cc1f1f648f17d9284e65f1ad69be647de3d3)
diff --git a/configure.ac b/configure.ac
index 4703b6a..d4c1fd4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1746,6 +1746,7 @@
 if test "x$enable_gallium_llvm" = xauto; then
     case "$host_cpu" in
     i*86|x86_64|amd64) enable_gallium_llvm=yes;;
+    *) enable_gallium_llvm=no;;
     esac
 fi