libbpf: remove linux/unaligned.h dependency for libbpf_sha256()

linux/unaligned.h include dependency is causing issues for libbpf's
Github mirror due to {get,put}_unaligned_be32() usage.

So get rid of it by implementing custom variants of those macros that
will work both in kernel and Github mirror repos.

Also switch round_up() to roundup(), as the former is not available in
Github mirror (and is just a subtly more specific variant of roundup()
anyways).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20251001171326.3883055-6-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>