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 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.