libbpf: define __uptr.

Make __uptr available to BPF programs to enable them to define uptrs.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/r/20241023234759.860539-8-martin.lau@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>