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: 8859b0da5a ("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

tools/scripts/Makefile.arch

@@ -7,8 +7,8 @@ HOSTARCH := $(shell uname -m | sed -e s/i.86/x86/ -e s/x86_64/x86/ \
                                   -e s/sh[234].*/sh/ -e s/aarch64.*/arm64/ \
                                   -e s/riscv.*/riscv/ -e s/loongarch.*/loongarch/)
 
-ifndef ARCH
-ARCH := $(HOSTARCH)
+ifeq ($(strip $(ARCH)),)
+override ARCH := $(HOSTARCH)
 endif
 
 SRCARCH := $(ARCH)