This is a bit ugly as we need to assume the structure of the C stack as
left by StgRun. Nevertheless, it allows us to unwind all the way back to
`_start` on my machine.

```
Stack trace:
    set_initial_registers (rts/Libdw.c:272.0)
    dwfl_thread_getframes

    dwfl_getthreads
    dwfl_getthread_frames
    libdw_get_backtrace (rts/Libdw.c:243.0)
    base_GHCziExecutionStack_getStackTrace1_info
(libraries/base/GHC/ExecutionStack.hs:43.1)
    base_GHCziExecutionStack_showStackTrace1_info
(libraries/base/GHC/ExecutionStack.hs:47.1)
    base_GHCziBase_bindIO1_info (libraries/base/GHC/Base.hs:1085.1)
    base_GHCziBase_thenIO1_info (libraries/base/GHC/Base.hs:1088.1)
    base_GHCziBase_thenIO1_info (libraries/base/GHC/Base.hs:1088.1)
    base_GHCziBase_thenIO1_info (libraries/base/GHC/Base.hs:1088.1)
    base_GHCziBase_thenIO1_info (libraries/base/GHC/Base.hs:1088.1)
    base_GHCziBase_thenIO1_info (libraries/base/GHC/Base.hs:1088.1)
    stg_catch_frame_info (rts/Exception.cmm:370.1)
    stg_stop_thread_info (rts/StgStartup.cmm:42.1)
    scheduleWaitThread (rts/Schedule.c:465.0)
    hs_main (rts/RtsMain.c:65.0)

    __libc_start_main (/tmp/buildd/glibc-2.19/csu/libc-start.c:321.0)
    _start
```