meson: rename "arch" variable

Avoid confusion between the ARCH variable of configure/config-host.mak
and the same-named variable of meson.build.

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>