gdb: add "unwinder class" to frame unwinders

A future patch will add a way to disable certain unwinders based on
different characteristics. This patch aims done to make it more
convenient to disable related unwinders in bulk, such as architecture
specific ones, by indentifying all unwinders by which part of the code
adds it. The classes, and explanations, are as follows:

* GDB: An internal unwinder, added by GDB core, such as the unwinder
  for dummy frames;
* EXTENSION: Unwinders added by extension languages;
* DEBUGINFO: Unwinders installed by the debug info reader;
* ARCH: Unwinders installed by the architecture specific code.