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 <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Tested-by: Alexandre Ghiti <[e-mail]>
Reviewed-by: Jean-Philippe Brucker <[e-mail]>
Reviewed-by: Namhyung Kim <[e-mail]>
Reviewed-by: Toke Høiland-Jørgensen <[e-mail]>
Acked-by: Quentin Monnet <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Cc: Arnaldo Carvalho de Melo <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/scripts/Makefile.arch

···
                                   -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)