selftests/bpf: Add tests for BPF_END bitwise tracking

selftests/bpf: Add tests for BPF_END bitwise tracking

Now BPF_END has bitwise tracking support. This patch adds selftests to
cover various cases of BPF_END (`bswap(16|32|64)`, `be(16|32|64)`,
`le(16|32|64)`) with bitwise propagation.

This patch is based on existing `verifier_bswap.c`, and add several
types of new tests:

1. Unconditional byte swap operations:
   - bswap16/bswap32/bswap64 with unknown bytes

2. Endian conversion operations (architecture-aware):
   - be16/be32/be64: convert to big-endian
     * on little-endian: do swap
     * on big-endian: truncation (16/32-bit) or no-op (64-bit)
   - le16/le32/le64: convert to little-endian
     * on big-endian: do swap
     * on little-endian: truncation (16/32-bit) or no-op (64-bit)

Each test simulates realistic networking scenarios where a value is
masked with unknown bits (e.g., var_off=(0x0; 0x3f00), range=[0,0x3f00]),
then byte-swapped, and the verifier must prove the result stays within
expected bounds.

Specifically, these selftests are based on dead code elimination:
If the BPF verifier can precisely track bitwise through byte swap
operations, it can prune the trap path (invalid memory access) that
should be unreachable, allowing the program to pass verification.
If bitwise tracking is incorrect, the verifier cannot prove the trap
is unreachable, causing verification failure.

The tests use preprocessor conditionals (#ifdef __BYTE_ORDER__) to
verify correct behavior on both little-endian and big-endian
architectures, and require Clang 18+ for bswap instruction support.

Co-developed-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Signed-off-by: Shenghao Yuan <shenghaoyuan0928@163.com>
Co-developed-by: Yazhou Tang <tangyazhou518@outlook.com>
Signed-off-by: Yazhou Tang <tangyazhou518@outlook.com>
Signed-off-by: Tianci Cao <ziye@zju.edu.cn>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260204111503.77871-3-ziye@zju.edu.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>