use struct pt_regs * rather than void * for powerpc[64] sigcontext regs

this is to match the kernel and glibc interfaces. here, struct pt_regs
is an incomplete type, but that's harmless, and if it's completed by
inclusion of another header then members of the struct pointed to by
the regs member can be accessed directly without going through a cast
or intermediate pointer object.

arch/powerpc/bits/signal.h

@@ -28,7 +28,7 @@ struct sigcontext {
 	int signal;
 	unsigned long handler;
 	unsigned long oldmask;
-	void *regs;
+	struct pt_regs *regs;
 };
 
 typedef struct {

arch/powerpc64/bits/signal.h

@@ -32,7 +32,7 @@ typedef struct sigcontext {
 	int _pad0;
 	unsigned long handler;
 	unsigned long oldmask;
-	void *regs;
+	struct pt_regs *regs;
 	gregset_t gp_regs;
 	fpregset_t fp_regs;
 	vrregset_t *v_regs;