tools: Override makefile ARCH variable if defined, but empty

There are a number of tools (bpftool, selftests), that require a
"bootstrap" build. Here, a bootstrap build is a build host variant of
a target. E.g., assume that you're performing a bpftool cross-build on
x86 to riscv, a bootstrap build would then be an x86 variant of
bpftool. The typical way to perform the host build variant, is to pass
"ARCH=" in a sub-make. However, if a variable has been set with a
command argument, then ordinary assignments in the makefile are
ignored.

This side-effect results in that ARCH, and variables depending on ARCH
are not set. Workaround by overriding ARCH to the host arch, if ARCH
is empty.

Fixes: 8859b0da5aac ("tools/bpftool: Fix cross-build")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Acked-by: Quentin Monnet <qmo@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/bpf/20241127101748.165693-1-bjorn@kernel.org

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