Fix stack pointer definition on powerpc64.

Index: lib/sigsegv.c
--- lib/sigsegv.c.orig
+++ lib/sigsegv.c
@@ -610,9 +610,15 @@ int libsigsegv_version = LIBSIGSEGV_VERSION;
 # elif defined __powerpc__
 
 /* See the definition of 'struct sigcontext' and 'struct trapframe' in
-   openbsd-src/sys/arch/powerpc/include/signal.h.  */
+   openbsd-src/sys/arch/powerpc/include/signal.h
+   and the definition of 'struct sigcontext' in
+   openbsd-src/sys/arch/powerpc64/include/signal.h.  */
 
-#  define SIGSEGV_FAULT_STACKPOINTER  scp->sc_frame.fixreg[1]
+#  if defined __powerpc64__ /* 64-bit */
+#   define SIGSEGV_FAULT_STACKPOINTER  scp->sc_sp
+#  else /* 32-bit */
+#   define SIGSEGV_FAULT_STACKPOINTER  scp->sc_frame.fixreg[1]
+#  endif
 
 # elif defined __sh__
 
