selftest/bpf: Adapt inline asm operand constraint for GCC support

GCC errors when compiling tailcall_bpf2bpf_hierarchy2.c and
tailcall_bpf2bpf_hierarchy3.c with the following error:

progs/tailcall_bpf2bpf_hierarchy2.c: In function 'tailcall_bpf2bpf_hierarchy_2':
progs/tailcall_bpf2bpf_hierarchy2.c:66:9: error: input operand constraint contains '+'
66 | asm volatile (""::"r+"(ret));
| ^~~

Changed implementation to make use of __sink macro that abstracts the
desired behaviour.

The proposed change seems valid for both GCC and CLANG.

Signed-off-by: Cupertino Miranda <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy2.c

···
 SEC("tc")
 int tailcall_bpf2bpf_hierarchy_2(struct __sk_buff *skb)
 {
-	volatile int ret = 0;
+	int ret = 0;
 
 	subprog_tail0(skb);
 	subprog_tail1(skb);
 
-	asm volatile (""::"r+"(ret));
+	__sink(ret);
 	return (count1 << 16) | count0;
 }
 

tools/testing/selftests/bpf/progs/tailcall_bpf2bpf_hierarchy3.c

···
 SEC("tc")
 int tailcall_bpf2bpf_hierarchy_3(struct __sk_buff *skb)
 {
-	volatile int ret = 0;
+	int ret = 0;
 
 	bpf_tail_call_static(skb, &jmp_table0, 0);
 
-	asm volatile (""::"r+"(ret));
+	__sink(ret);
 	return ret;
 }