Fix build on powerpc64 by correcting the stack pointer definition

Index: lib/sigsegv.c
--- lib/sigsegv.c.orig
+++ lib/sigsegv.c
@@ -223,7 +223,6 @@ int libsigsegv_version = LIBSIGSEGV_VERSION;
    the 'struct sigcontext' defined in <asm/sigcontext.h>,
    and the 'struct pt_regs' defined in <asm/ptrace.h>
    are quite different types.  */
-
 #  if defined __powerpc64__ || defined __powerpc64_elfv2__ /* 64-bit */
 #   define SIGSEGV_FAULT_STACKPOINTER  ((ucontext_t *) ucp)->uc_mcontext.gp_regs[1]
 #  else /* 32-bit */
@@ -608,7 +607,11 @@ int libsigsegv_version = LIBSIGSEGV_VERSION;
 /* See the definition of 'struct sigcontext' and 'struct trapframe' in
    openbsd-src/sys/arch/powerpc/include/signal.h.  */
 
-#  define SIGSEGV_FAULT_STACKPOINTER  scp->sc_frame.fixreg[1]
+#  ifdef __powerpc64__
+#    define SIGSEGV_FAULT_STACKPOINTER scp->sc_sp
+#  else
+#    define SIGSEGV_FAULT_STACKPOINTER  scp->sc_frame.fixreg[1]
+#  endif
 
 # elif defined __sh__