Avoid cloning the stack on every `push_ctxt` call in trans

Rewrite the use of TLS variable for `push_ctxt` so that it uses a ~[]
instead of a @~[]. Before it cloned the whole vector on each push and
pop, which is unnecessary.
auto merge of #9723 : blake2-ppc/rust/trans-no-push-ctxt-clone, r=alexcrichton

Avoid cloning the stack on every `push_ctxt` call in trans

Rewrite the use of TLS variable for `push_ctxt` so that it uses a ~[]
instead of a @~[]. Before it cloned the whole vector on each push and
pop, which is unnecessary.