Mostly recover a Ruby stack trace from a core file

Update the lldb script so it can mostly recover a Ruby stack trace from
a core file.  It's still missing line numbers and dealing with CFUNCs,
but you use it like this:

```
(lldb) rbbt ec
rb_control_frame_t TYPE
0x7f6fd6555fa0     EVAL   ./bootstraptest/runner.rb error!!
0x7f6fd6555f68     METHOD ./bootstraptest/runner.rb main
0x7f6fd6555f30     METHOD ./bootstraptest/runner.rb in_temporary_working_directory
0x7f6fd6555ef8     METHOD /home/aaron/git/ruby/lib/tmpdir.rb mktmpdir
0x7f6fd6555ec0     BLOCK  ./bootstraptest/runner.rb block in in_temporary_working_directory
0x7f6fd6555e88     CFUNC
0x7f6fd6555e50     BLOCK  ./bootstraptest/runner.rb block (2 levels) in in_temporary_working_directory
0x7f6fd6555e18     BLOCK  ./bootstraptest/runner.rb block in main
0x7f6fd6555de0     METHOD ./bootstraptest/runner.rb exec_test
0x7f6fd6555da8     CFUNC
0x7f6fd6555d70     BLOCK  ./bootstraptest/runner.rb block in exec_test
0x7f6fd6555d38     CFUNC
0x7f6fd6555d00     TOP    /home/aaron/git/ruby/bootstraptest/test_insns.rb error!!
0x7f6fd6555cc8     CFUNC
0x7f6fd6555c90     BLOCK  /home/aaron/git/ruby/bootstraptest/test_insns.rb block in <top (required)>
0x7f6fd6555c58     METHOD ./bootstraptest/runner.rb assert_equal
0x7f6fd6555c20     METHOD ./bootstraptest/runner.rb assert_check
0x7f6fd6555be8     METHOD ./bootstraptest/runner.rb show_progress
0x7f6fd6555bb0     METHOD ./bootstraptest/runner.rb with_stderr
0x7f6fd6555b78     BLOCK  ./bootstraptest/runner.rb block in show_progress
0x7f6fd6555b40     BLOCK  ./bootstraptest/runner.rb block in assert_check
0x7f6fd6555b08     METHOD ./bootstraptest/runner.rb get_result_string
0x7f6fd6555ad0     METHOD ./bootstraptest/runner.rb make_srcfile
0x7f6fd6555a98     CFUNC
0x7f6fd6555a60     BLOCK  ./bootstraptest/runner.rb block in make_srcfile
```

Getting the main execution context is difficult (it is stored in a
thread local) so for now you must supply an ec and this will make a
backtrace

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