selftests/bpf: Adapt OUTPUT appending logic to lower versions of Make

The $(let ...) function is only supported by GNU Make version 4.4 [0]
and above, otherwise the following exception file or directory will be
generated:

	tools/testing/selftests/bpfFEATURE-DUMP.selftests
	tools/testing/selftests/bpffeature/

Considering that the GNU Make version of most Linux distributions is
lower than 4.4, let us adapt the corresponding logic to it.

Link: https://lists.gnu.org/archive/html/info-gnu/2022-10/msg00008.html

 [0]
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20240905081401.1894789-2-pulehui@huaweicloud.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>