sched: split sched.h into separate header files

arch/x86_64/asm-offset.c

@@ -1,10 +1,11 @@
-#include <kernel/sched.h>
 #include <kernel/compiler.h>
+#include <kernel/sched.h>
+#include <kernel/thread.h>
 
-//size_t THREAD_sp = offsetof(struct thread, tr_sp);
+// size_t THREAD_sp = offsetof(struct thread, tr_sp);
 
-/* Use %a0 instead of %0 to prevent gcc from emitting a $ before the symbol value
-   in the generated assembly.
+/* Use %a0 instead of %0 to prevent gcc from emitting a $ before the symbol
+   value in the generated assembly.
 
    emitting
      .set TASK_sp, $56