Use $cpu instead of $ARCH, which has been removed from
the top-level configure.

Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Fixes: 823eb013 ("configure, meson: move ARCH to meson.build")
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>