selftests/bpf: Cross-compile bpftool

When the BPF selftests are cross-compiled, only the a host version of
bpftool is built. This version of bpftool is used on the host-side to
generate various intermediates, e.g., skeletons.

The test runners are also using bpftool, so the Makefile will symlink
bpftool from the selftest/bpf root, where the test runners will look
the tool:

  | $(Q)ln -sf $(if $2,..,.)/tools/build/bpftool/bootstrap/bpftool \
  |    $(OUTPUT)/$(if $2,$2/)bpftool

There are two problems for cross-compilation builds:

 1. There is no native (cross-compilation target) of bpftool
 2. The bootstrap/bpftool is never cross-compiled (by design)

Make sure that a native/cross-compiled version of bpftool is built,
and if CROSS_COMPILE is set, symlink the native/non-bootstrap version.

Acked-by: Quentin Monnet <quentin@isovalent.com>
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Link: https://lore.kernel.org/r/20230214161253.183458-1-bjorn@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

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