auto merge of #5887 : jdm/rust/stackbounds, r=brson

This is needed to allow GC to work in SpiderMonkey.

diff --cc src/libcore/gc.rs
Simple merge

diff --cc src/rt/rust_builtin.cpp
Simple merge

diff --cc src/rt/rust_task.h
Simple merge

diff --cc src/rt/rustrt.def.in
index cfc7abac6eb816b3228897c42f5dcddb7126163f,b7ca28d5960da2a9e1fd8f28c1489ba113ba9c05..977e0248ca206dcc535ec58cf5d3d82226a3323a
--- 1/src/rt/rustrt.def.in
--- 2/src/rt/rustrt.def.in
+++ b/src/rt/rustrt.def.in
@@@ -47,8 -47,8 +47,9 @@@ rust_env_pair
  rust_task_yield
  rust_task_is_unwinding
  rust_get_task
 +rust_try_get_task
  rust_get_stack_segment
+ rust_get_c_stack
  rust_log_str
  start_task
  vec_reserve_shared_actual