Merge branch 'selftests/bpf: Some build fixes'

Jean-Philippe Brucker says:

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A few fixes for cross-building the sefltests out of tree. This will
enable wider automated testing on various Arm hardware.

Changes since v1 [1]:
* Use wildcard in patch 5
* Move the MAKE_DIRS declaration in patch 1

[1] https://lore.kernel.org/bpf/20210112135959.649075-1-jean-philippe@linaro.org/
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Signed-off-by: Andrii Nakryiko <andrii@kernel.org>