[dev.power64] os/signal: support for linux/power64 and linux/power64le

LGTM=rsc
R=rsc, iant
CC=golang-codereviews
https://golang.org/cl/121580043

src/pkg/os/signal/sig.s

@@ -4,13 +4,19 @@
 
 // Assembly to get into package runtime without using exported symbols.
 
-// +build amd64 amd64p32 arm 386
+// +build amd64 amd64p32 arm 386 power64 power64le
 
 #include "../../../cmd/ld/textflag.h"
 
 #ifdef GOARCH_arm
 #define JMP B
 #endif
+#ifdef GOARCH_power64
+#define JMP BR
+#endif
+#ifdef GOARCH_power64le
+#define JMP BR
+#endif
 
 TEXT ·signal_disable(SB),NOSPLIT,$0
 	JMP runtime·signal_disable(SB)