bpftool: Add support for KIND_RESTRICT to gen min_core_btf command

This change adjusts bpftool's type marking logic, as used in conjunction
with TYPE_EXISTS relocations, to correctly recognize and handle the
RESTRICT BTF kind.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Müller <deso@posteo.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Quentin Monnet <quentin@isovalent.com>
Link: https://lore.kernel.org/bpf/20220623212205.2805002-1-deso@posteo.net/T/#m4c75205145701762a4b398e0cdb911d5b5305ffc
Link: https://lore.kernel.org/bpf/20220706212855.1700615-2-deso@posteo.net