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/loop1.c

@@ -18,7 +18,7 @@ int nested_loops(volatile struct pt_regs* ctx)
 	for (j = 0; j < 300; j++)
 		for (i = 0; i < j; i++) {
 			if (j & 1)
-				m = ctx->rax;
+				m = PT_REGS_RC(ctx);
 			else
 				m = j;
 			sum += i * m;

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;

tools/testing/selftests/bpf/progs/loop3.c

@@ -16,7 +16,7 @@ int while_true(volatile struct pt_regs* ctx)
 	__u64 i = 0, sum = 0;
 	do {
 		i++;
-		sum += ctx->rax;
+		sum += PT_REGS_RC(ctx);
 	} while (i < 0x100000000ULL);
 	return sum;
 }