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 <[e-mail]>
Signed-off-by: Pu Lehui <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/Makefile

···
 FEATURE_DISPLAY := $(FEATURE_TESTS)
 
 # Makefile.feature expects OUTPUT to end with a slash
+ifeq ($(shell expr $(MAKE_VERSION) \>= 4.4), 1)
 $(let OUTPUT,$(OUTPUT)/,\
 	$(eval include ../../../build/Makefile.feature))
+else
+OUTPUT := $(OUTPUT)/
+$(eval include ../../../build/Makefile.feature)
+OUTPUT := $(patsubst %/,%,$(OUTPUT))
+endif
 endif
 
 ifeq ($(feature-llvm),1)