selftest/bpf: Replace magic constants by macros

Replace magic constants in a BTF structure initialization code by
proper macros, as is done in other similar selftests.

Suggested-by: Eduard Zingerman <[e-mail]>
Signed-off-by: Anton Protopopov <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/progs/syscall.c

···
 			.magic = BTF_MAGIC,
 			.version = BTF_VERSION,
 			.hdr_len = sizeof(struct btf_header),
-			.type_len = sizeof(__u32) * 8,
-			.str_off = sizeof(__u32) * 8,
-			.str_len = sizeof(__u32),
+			.type_len = sizeof(raw_btf.types),
+			.str_off = offsetof(struct btf_blob, str) - offsetof(struct btf_blob, types),
+			.str_len = sizeof(raw_btf.str),
 		},
 		.types = {
 			/* long */