runtime: add Callers

cut copies of traceback from 6 to 1.

R=r
CC=golang-dev
https://golang.org/cl/703041

src/pkg/runtime/runtime.h

diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 194503e..622f680 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -407,6 +407,7 @@
 void	siginit(void);
 bool	sigsend(int32 sig);
 void	gettime(int64*, int32*);
+int32	callers(int32, uintptr*, int32);
 int64	nanotime(void);
 
 #pragma	varargck	argpos	printf	1