DWARF 5 standardized the .gnu_debugaltlink section that dwz emits in
multi-file mode.  This is handled via some new forms, and a new
.debug_sup section.

This patch adds support for this to gdb.  It is largely
straightforward, I think, though one oddity is that I chose not to
have this code search the system build-id directories for the
supplementary file.  My feeling was that, while it makes sense for a
distro to unify the build-id concept with the hash stored in the
.debug_sup section, there's no intrinsic need to do so.

This in turn means that a few tests -- for example those that test the
index cache -- will not work in this mode.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32808