selftests/bpf: Test the inlining of bpf_kptr_xchg()

The test uses bpf_prog_get_info_by_fd() to obtain the xlated
instructions of the program first. Since these instructions have
already been rewritten by the verifier, the tests then checks whether
the rewritten instructions are as expected. And to ensure LLVM generates
code exactly as expected, use inline assembly and a naked function.

Suggested-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240105104819.3916743-4-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>