sched: Have sched_class_highest define by vmlinux.lds.h

Now that the sched_class descriptors are defined by the linker script, and
this needs to be aware of the existance of stop_sched_class when SMP is
enabled or not, as it is used as the "highest" priority when defined. Move
the declaration of sched_class_highest to the same location in the linker
script that inserts stop_sched_class, and this will also make it easier to
see what should be defined as the highest class, as this linker script
location defines the priorities as well.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20191219214558.682913590@goodmis.org

include/asm-generic/vmlinux.lds.h

@@ -114,11 +114,14 @@
  * relation to each other.
  */
 #define SCHED_DATA				\
+	STRUCT_ALIGN();				\
+	__begin_sched_classes = .;		\
 	*(__idle_sched_class)			\
 	*(__fair_sched_class)			\
 	*(__rt_sched_class)			\
 	*(__dl_sched_class)			\
-	*(__stop_sched_class)
+	*(__stop_sched_class)			\
+	__end_sched_classes = .;
 
 /*
  * Align to a 32 byte boundary equal to the