Add 'bench' target to makefiles.

R=rsc, agl, r
CC=golang-dev
https://golang.org/cl/188043
diff --git a/src/Make.pkg b/src/Make.pkg
index d4196b6..3dd57b7 100644
--- a/src/Make.pkg
+++ b/src/Make.pkg
@@ -53,6 +53,9 @@
 test:
 	$(QUOTED_GOBIN)/gotest
 
+bench:
+	$(QUOTED_GOBIN)/gotest -benchmarks=. -match="Do not run tests"
+
 nuke: clean
 	rm -f $(pkgdir)/$(TARG).a