selftests/bpf: Workaround for llvm nop-4 bug

Currently LLVM fails to recognize .data.* as data section and defaults to .text
section. Later BPF backend tries to emit 4-byte NOP instruction which doesn't
exist in BPF ISA and aborts.
The fix for LLVM is pending:
https://reviews.llvm.org/D138477



While waiting for the fix lets workaround the linked_list test case
by using .bss.* prefix which is properly recognized by LLVM as BSS section.

Fix libbpf to support .bss. prefix and adjust tests.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>