runtime: fix tiny memory leak

The m->cret word holds the C return value when returning
across a stack split boundary.  It was not being cleared after
use, which means that the return value (if a C function)
or else the value of AX/R0 at the time of the last stack unsplit
was being kept alive longer than necessary.  Clear it.

I think the effect here should be very small, but worth fixing
anyway.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5677092

src/pkg/runtime/proc.c

@@ -1011,6 +1011,7 @@ runtime·oldstack(void)
 {
 	Stktop *top, old;
 	uint32 argsize;
+	uintptr cret;
 	byte *sp;
 	G *g1;
 	int32 goid;
@@ -1034,7 +1035,9 @@ runtime·oldstack(void)
 	g1->stackbase = old.stackbase;
 	g1->stackguard = old.stackguard;
 
-	runtime·gogo(&old.gobuf, m->cret);
+	cret = m->cret;
+	m->cret = 0;  // drop reference
+	runtime·gogo(&old.gobuf, cret);
 }
 
 // Called from reflect·call or from runtime·morestack when a new