Introduce type-safe variant of gdb_bfd_openr_iovec

Introduce type-safe variant of gdb_bfd_openr_iovec

This patch adds a new, type-safe variant of gdb_bfd_openr_iovec.  In
this approach, the underlying user data is simply an object, the
callbacks are methods, and the "open" function is a function view.
Nothing uses this new code yet.

Reviewed-By: Lancelot Six <lancelot.six@amd.com>