all: power64 is now ppc64

Fixes #8654.

LGTM=austin
R=austin
CC=golang-codereviews
https://golang.org/cl/180600043
diff --git a/src/runtime/defs3_linux.go b/src/runtime/defs3_linux.go
index 3551a4f..489c130 100644
--- a/src/runtime/defs3_linux.go
+++ b/src/runtime/defs3_linux.go
@@ -7,7 +7,7 @@
 /*
 Input to cgo -cdefs
 
-GOARCH=power64 cgo -cdefs defs_linux.go defs3_linux.go > defs_linux_power64.h
+GOARCH=ppc64 cgo -cdefs defs_linux.go defs3_linux.go > defs_linux_ppc64.h
 */
 
 package runtime