rt::task: Make current_stack_segment public again

This was done in 2145de8c and reverted in 0ada7c7f, but Servo needs it.

Closes #10065.
auto merge of #10141 : kmcallister/rust/current_stack_segment, r=alexcrichton

This was done in 2145de8c and reverted in 0ada7c7f, but Servo needs it.

Closes #10065.

r? @brson