selftests/bpf: match both retq/rethunk in verifier_tailcall_jit

Depending on kernel parameters, x86 jit generates either retq or jump
to rethunk for 'exit' instruction. The difference could be seen when
kernel is booted with and without mitigations=off parameter.
Relax the verifier_tailcall_jit test case to match both variants.

Fixes: e5bdd6a ("selftests/bpf: validate jit behaviour for tail calls")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240823080644.263943-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: eddyz87

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

@@ -59,7 +59,7 @@ __jited("	movq	-0x10(%rbp), %rax")
 __jited("	callq	0x{{.*}}")		/* call to sub()          */
 __jited("	xorl	%eax, %eax")
 __jited("	leave")
-__jited("	retq")
+__jited("	{{(retq|jmp	0x)}}")		/* return or jump to rethunk */
 __jited("...")
 /* subprogram entry for sub(), regular function prologue */
 __jited("	endbr64")
@@ -89,7 +89,7 @@ __jited("	popq	%rax")
 __jited("	popq	%rax")
 __jited("	jmp	{{.*}}")		/* jump to tail call tgt   */
 __jited("L0:	leave")
-__jited("	retq")
+__jited("	{{(retq|jmp	0x)}}")		/* return or jump to rethunk */
 SEC("tc")
 __naked int main(void)
 {