runtime: efence support for growable stacks

1. Fix the bug that shrinkstack returns memory to heap.
   This causes growslice to misbehave (it manually initialized
   blocks, and in efence mode shrinkstack's free leads to
   partially-initialized blocks coming out of growslice.
   Which in turn causes GC to crash while treating the garbage
   as Eface/Iface.
2. Enable efence for stack segments.

LGTM=rsc
R=golang-codereviews, rsc
CC=golang-codereviews, khr
https://golang.org/cl/74080043

src/pkg/runtime/stack.c

@@ -102,7 +102,7 @@ runtime·stackalloc(G *gp, uint32 n)
 		runtime·printf("stackalloc %d\n", n);
 
 	gp->stacksize += n;
-	if(StackFromSystem)
+	if(runtime·debug.efence || StackFromSystem)
 		return runtime·SysAlloc(ROUND(n, PageSize), &mstats.stacks_sys);
 
 	// Minimum-sized stacks are allocated with a fixed-size free-list allocator,
@@ -143,8 +143,8 @@ runtime·stackfree(G *gp, void *v, Stktop *top)
 	if(StackDebug >= 1)
 		runtime·printf("stackfree %p %d\n", v, (int32)n);
 	gp->stacksize -= n;
-	if(StackFromSystem) {
-		if(StackFaultOnFree)
+	if(runtime·debug.efence || StackFromSystem) {
+		if(runtime·debug.efence || StackFaultOnFree)
 			runtime·SysFault(v, n);
 		else
 			runtime·SysFree(v, n, &mstats.stacks_sys);
@@ -819,7 +819,15 @@ runtime·shrinkstack(G *gp)
 		gp->stack0 = (uintptr)oldstk + newsize;
 	gp->stacksize -= oldsize - newsize;
 
-	// Free bottom half of the stack.  First, we trick malloc into thinking
+	// Free bottom half of the stack.
+	if(runtime·debug.efence || StackFromSystem) {
+		if(runtime·debug.efence || StackFaultOnFree)
+			runtime·SysFault(oldstk, newsize);
+		else
+			runtime·SysFree(oldstk, newsize, &mstats.stacks_sys);
+		return;
+	}
+	// First, we trick malloc into thinking
 	// we allocated the stack as two separate half-size allocs.  Then the
 	// free() call does the rest of the work for us.
 	if(oldsize == PageSize) {