tools, bpftool: Poison and replace kernel integer typedefs

Replace the use of kernel-only integer typedefs (u8, u32, etc.) by their
user space counterpart (__u8, __u32, etc.).

Similarly to what libbpf does, poison the typedefs to avoid introducing
them again in the future.

Signed-off-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20200511161536.29853-2-quentin@isovalent.com