liburcu: Switch from @!arc to targets

Works around a bug with the buildbots where they try to build it.

Signed-off-by: Rosen Penev <rosenp@gmail.com>

libs/liburcu/Makefile

@@ -32,7 +32,7 @@ define Package/liburcu
 	CATEGORY:=Libraries
 	TITLE:=User-space Read-Copy-Update library
 	URL:=https://lttng.org/
-	DEPENDS:=+libpthread @!arc
+	DEPENDS:=+libpthread @!TARGET_arc770 @1TARGET_archs38
 endef
 
 define Package/liburcu/description