Expose stopped cpu contexts to ddb on PowerPC

Summary: In r220638, stoppcbs started being tracked. This never got exposed to
ddb though, so kdb_thr_ctx() didn't know how to look them up.

This allows switching to threads on stopped CPUs in kdb.

Submitted by:	Brandon Bergren <git_bdragon.rkt0.net>
Differential Revision: https://reviews.freebsd.org/D15986

sys/powerpc/include/kdb.h

@@ -40,6 +40,8 @@
 void kdb_cpu_clear_singlestep(void);
 void kdb_cpu_set_singlestep(void);
 
+#define KDB_STOPPEDPCB(pc)      &stoppcbs[pc->pc_cpuid]
+
 static __inline void
 kdb_cpu_sync_icache(unsigned char *addr, size_t size)
 {

sys/powerpc/include/smp.h

@@ -42,6 +42,7 @@
 
 #ifndef LOCORE
 
+#include <machine/pcb.h>
 #include <sys/_cpuset.h>
 
 void	ipi_all_but_self(int ipi);
@@ -59,6 +60,8 @@ uintptr_t cpudep_ap_bootstrap(void);
 void	cpudep_ap_setup(void);
 void	machdep_ap_bootstrap(void);
 
+extern struct pcb stoppcbs[];
+
 #endif /* !LOCORE */
 #endif /* _KERNEL */
 #endif /* !_MACHINE_SMP_H */