Change the setup so that rust_activate_glue returns to rust_exit_task_glu #348
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Change the setup so that rust_activate_glue returns to rust_exit_task_glue
and rust_exit_task_glue calls the rust main.
This is simpler since we only need to setup one frame. It also matches
what ld.so does, so gdb is happy and stops a backtrace at rust_exit_task_glue
instead of continuing past whatever function happened to be before
rust_exit_task_glue is the object file.