runtime: instrument malloc + garbage collector.
add simple garbage collection benchmark.

R=iant
CC=golang-dev
https://golang.org/cl/204053
diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 2d6d42e..a526c04 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -393,6 +393,8 @@
 void	·newproc(int32, byte*, byte*);
 void	siginit(void);
 bool	sigsend(int32 sig);
+void	gettime(int64*, int32*);
+int64	nanotime(void);
 
 #pragma	varargck	argpos	printf	1