[dev.power64] runtime: add defs3_linux.go to generate defs_linux_power64{,le}.h

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/128070043
diff --git a/src/pkg/runtime/defs1_linux.go b/src/pkg/runtime/defs1_linux.go
index 451817a..c0b510a 100644
--- a/src/pkg/runtime/defs1_linux.go
+++ b/src/pkg/runtime/defs1_linux.go
@@ -15,12 +15,14 @@
 /*
 #include <ucontext.h>
 #include <fcntl.h>
+#include <asm/signal.h>
 */
 import "C"
 
 const (
-	O_RDONLY  = C.O_RDONLY
-	O_CLOEXEC = C.O_CLOEXEC
+	O_RDONLY    = C.O_RDONLY
+	O_CLOEXEC   = C.O_CLOEXEC
+	SA_RESTORER = C.SA_RESTORER
 )
 
 type Usigset C.__sigset_t