[kernel] tweak a few thread apis to to take a const pointer

A bit of reformatting on some ARM code while was touching it.

arch/m68k/thread.c

@@ -53,7 +53,7 @@ void arch_context_switch(thread_t *oldthread, thread_t *newthread) {
     m68k_context_switch(&oldthread->arch.cs_frame, &newthread->arch.cs_frame);
 }
 
-void arch_dump_thread(thread_t *t) {
+void arch_dump_thread(const thread_t *t) {
 #if 0
     if (t->state != THREAD_RUNNING) {
         dprintf(INFO, "\tarch: ");