Merge branch 'tools/bpftool: Some build fixes'

Jean-Philippe Brucker says:

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A few fixes for cross and out-of-tree build of bpftool and runqslower.
These changes allow to build for different target architectures, using
the same source tree.

Since [v2], I addressed Andrii's comments on patches 3 and 5, and added
patch 7 which fixes a build slowdown.

[v2] https://lore.kernel.org/bpf/20201109110929.1223538-1-jean-philippe@linaro.org/
====================

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>