selftests/bpf: Fix symlink creation error

When building with O=, the following error occurs:

    ln: failed to create symbolic link 'no_alu32/bpftool': No such file or directory

Adjust the code to account for $(OUTPUT).
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20230128000650.1516334-6-iii@linux.ibm.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/Makefile

@@ -519,7 +519,8 @@ $(OUTPUT)/$(TRUNNER_BINARY): $(TRUNNER_TEST_OBJS)			\
 	$$(call msg,BINARY,,$$@)
 	$(Q)$$(CC) $$(CFLAGS) $$(filter %.a %.o,$$^) $$(LDLIBS) -o $$@
 	$(Q)$(RESOLVE_BTFIDS) --btf $(TRUNNER_OUTPUT)/btf_data.bpf.o $$@
-	$(Q)ln -sf $(if $2,..,.)/tools/build/bpftool/bootstrap/bpftool $(if $2,$2/)bpftool
+	$(Q)ln -sf $(if $2,..,.)/tools/build/bpftool/bootstrap/bpftool \
+		   $(OUTPUT)/$(if $2,$2/)bpftool
 
 endef