runtime: run deferred calls at Goexit

baby step toward panic+recover.

Fixes #349.

R=r
CC=golang-dev
https://golang.org/cl/825043
diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index d20d5b9..2671a05 100644
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -239,6 +239,7 @@
 	// fp == gobuf.sp except in the case of a reflected
 	// function call, which uses an off-stack argument frame.
 	uint8*	fp;
+	bool	free;	// call stackfree for this frame?
 };
 struct	Alg
 {