Convert all_bfds to new hash table

Convert all_bfds to new hash table

This converts gdb_bfd.c to use the new hash table for all_bfds.

This patch slightly changes the htab_t pretty-printer test, which was
relying on all_bfds.  Note that with the new hash table, gdb-specific
printers aren't needed; the libstdc++ printers suffice -- in fact,
they are better, because the true types of the contents are available.

Change-Id: I48b7bd142085287b34bdef8b6db5587581f94280
Co-Authored-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>