kbuild: rpm-pkg: define _arch conditionally

Commit 3089b2be ("kbuild: rpm-pkg: fix build error when _arch is
undefined") does not work as intended; _arch is always defined as
$UTS_MACHINE.

The intention was to define _arch to $UTS_MACHINE only when it is not
defined.

Fixes: 3089b2be

 ("kbuild: rpm-pkg: fix build error when _arch is undefined")
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>