selftests/bpf: Avoid generating untracked files when running bpf selftests

Currently, when we run the BPF selftests with the following command:

make -C tools/testing/selftests TARGETS=bpf SKIP_TARGETS=""

The command generates untracked files and directories with make version
less than 4.4:

'''
Untracked files:
(use "git add <file>..." to include in what will be committed)
tools/testing/selftests/bpfFEATURE-DUMP.selftests
tools/testing/selftests/bpffeature/
'''

We lost slash after word "bpf". The reason is slash appending code is as
follow:

'''
OUTPUT := $(OUTPUT)/
$(eval include ../../../build/Makefile.feature)
OUTPUT := $(patsubst %/,%,$(OUTPUT))
'''

This way of assigning values to OUTPUT will never be effective for the
variable OUTPUT provided via the command argument [1] and BPF makefile
is called from parent Makfile(tools/testing/selftests/Makefile) like:

'''
all:
...
$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET
'''

According to GNU make, we can use override Directive to fix this issue [2].

[1] https://www.gnu.org/software/make/manual/make.html#Overriding
[2] https://www.gnu.org/software/make/manual/make.html#Override-Directive

Fixes: dc3a8804d790 ("selftests/bpf: Adapt OUTPUT appending logic to lower versions of Make")
Signed-off-by: Jiayuan Chen <mrpre@163.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20241224075957.288018-1-mrpre@163.com