libbpf: Fix undefined behavior in {get,put}_unaligned_be32()

These violate aliasing rules and may be miscompiled unless
-fno-strict-aliasing is used.  Replace them with the standard memcpy()
solution.  Note that compilers know how to optimize this properly.

Fixes: 4a1c9e54 ("libbpf: remove linux/unaligned.h dependency for libbpf_sha256()")
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20251006012037.159295-1-ebiggers@kernel.org