selftests/bpf: Test multi_st_ops and calling kfuncs from different programs

Test multi_st_ops and demonstrate how different bpf programs can call
a kfuncs that refers to the struct_ops instance in the same source file
by id. The id is defined as a global vairable and initialized before
attaching the skeleton. Kfuncs that take the id can hide the argument
with a macro to make it almost transparent to bpf program developers.

The test involves two struct_ops returning different values from
.test_1. In syscall and tracing programs, check if the correct value is
returned by a kfunc that calls .test_1.

Signed-off-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250806162540.681679-4-ameryhung@gmail.com