selftests/bpf: fix compiling loop{1, 2, 3}.c on s390

Use PT_REGS_RC(ctx) instead of ctx->rax, which is not present on s390.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Tested-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/progs/loop2.c

@@ -16,7 +16,7 @@ int while_true(volatile struct pt_regs* ctx)
 	int i = 0;
 
 	while (true) {
-		if (ctx->rax & 1)
+		if (PT_REGS_RC(ctx) & 1)
 			i += 3;
 		else
 			i += 7;