rt: Delete the entire stack chain on task destruction

Unwinding through __morestack on 64-bit Linux seems to be no big deal, and
all we have to do is free the stacks to make unwinding work with split stacks.

src/rt/rust_task.cpp

@@ -178,8 +178,11 @@ rust_task::~rust_task()
     I(sched, ref_count == 0); // ||
     //   (ref_count == 1 && this == sched->root_task));
 
+    // Delete all the stacks. There may be more than one if the task failed
+    while (stk != NULL) {
         del_stk(this, stk);
     }
+}
 
 struct spawn_args {
     rust_task *task;

src/test/run-fail/morestack1.rs

@@ -1,5 +1,6 @@
 // xfail-test
 // compile-flags:--stack-growth
+// error-pattern:explicit failure
 fn getbig(i: int) {
     if i != 0 {
         getbig(i - 1);
@@ -9,5 +10,5 @@ fn getbig(i: int) {
 }
 
 fn main() {
-    getbig(10000000);
+    getbig(100000);
 }