selftests/bpf: Add CO-RE relos and SEC("?...") to linked_funcs selftests

Enhance linked_funcs selftest with two tricky features that might not
obviously work correctly together. We add CO-RE relocations to entry BPF
programs and mark those programs as non-autoloadable with SEC("?...")
annotation. This makes sure that libbpf itself handles .BTF.ext CO-RE
relocation data matching correctly for SEC("?...") programs, as well as
ensures that BPF static linker handles this correctly (this was the case
before, no changes are necessary, but it wasn't explicitly tested).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220426004511.2691730-6-andrii@kernel.org

selftests/bpf: Add function linking selftest

Add selftest validating various aspects of statically linking functions:
- no conflicts and correct resolution for name-conflicting static funcs;
- correct resolution of extern functions;
- correct handling of weak functions, both resolution itself and libbpf's
handling of unused weak function that "lost" (it leaves gaps in code with
no ELF symbols);
- correct handling of hidden visibility to turn global function into
"static" for the purpose of BPF verification.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210423181348.1801389-16-andrii@kernel.org