gdb: make gdbarch store a vector of frame unwinders

Before this commit, all frame unwinders would be stored in the obstack
of a gdbarch and accessed by using the registry system. This made for
unwieldy code, and unnecessarily complex logic in the frame_unwinder
implementation, along with making frame_unwind structs be unable to have
non-trivial constructors.

Seeing as a future patch of this series wants to refactor the
frame_unwind struct to use inheritance, obstack storage would no longer
be viable. In preparation for that change, this commit adds an
std::vector to gdbarch to store the unwinders in.

There should be no user-visible changes.