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 <[e-mail]>

tools/lib/bpf/libbpf.c

···
 			sec_desc->sec_type = SEC_RELO;
 			sec_desc->shdr = sh;
 			sec_desc->data = data;
-		} else if (sh->sh_type == SHT_NOBITS && strcmp(name, BSS_SEC) == 0) {
+		} else if (sh->sh_type == SHT_NOBITS && (strcmp(name, BSS_SEC) == 0 ||
+							 str_has_pfx(name, BSS_SEC "."))) {
 			sec_desc->sec_type = SEC_BSS;
 			sec_desc->shdr = sh;
 			sec_desc->data = data;

tools/testing/selftests/bpf/prog_tests/linked_list.c

···
 	ASSERT_OK(ret, "global_list_push_pop");
 	ASSERT_OK(opts.retval, "global_list_push_pop retval");
 	if (!leave_in_map)
-		clear_fields(skel->maps.data_A);
+		clear_fields(skel->maps.bss_A);
 
 	if (mode == PUSH_POP)
 		goto end;
···
 	ASSERT_OK(ret, "global_list_push_pop_multiple");
 	ASSERT_OK(opts.retval, "global_list_push_pop_multiple retval");
 	if (!leave_in_map)
-		clear_fields(skel->maps.data_A);
+		clear_fields(skel->maps.bss_A);
 
 	if (mode == PUSH_POP_MULT)
 		goto end;
···
 	ASSERT_OK(ret, "global_list_in_list");
 	ASSERT_OK(opts.retval, "global_list_in_list retval");
 	if (!leave_in_map)
-		clear_fields(skel->maps.data_A);
+		clear_fields(skel->maps.bss_A);
 end:
 	linked_list__destroy(skel);
 }

tools/testing/selftests/bpf/progs/linked_list.h

···
 	},
 };
 
-#define private(name) SEC(".data." #name) __hidden __attribute__((aligned(8)))
+#define private(name) SEC(".bss." #name) __hidden __attribute__((aligned(8)))
 
 private(A) struct bpf_spin_lock glock;
 private(A) struct bpf_list_head ghead __contains(foo, node);