util/debug: Make debug_backtrace_capture work for 64bit windows.

Rely on Windows' CaptureStackBackTrace to do the grunt work.

Reviewed-by: Brian Paul <brianp@vmware.com>
util/debug: Cleanup/improve debug_symbol_name_dbghelp.

- use mgwhelp -- the successor for bfdhelp which does not have a hard
  dependency on BFD, and works on 64bits.
- use a macro instead of hand-typing to dispatch DbgHelp functions
- dump line numbers
- dump module names when symbols are not available
- support 64bits.
- add comments

Reviewed-by: Brian Paul <brianp@vmware.com>