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>

scripts/package/mkspec

@@ -57,7 +57,7 @@ $S	BuildRequires: gcc make openssl openssl-devel perl python3 rsync
 
 	# $UTS_MACHINE as a fallback of _arch in case
 	# /usr/lib/rpm/platform/*/macros was not included.
-	%define _arch %{?_arch:$UTS_MACHINE}
+	%{!?_arch: %define _arch $UTS_MACHINE}
 	%define __spec_install_post /usr/lib/rpm/brp-compress || :
 	%define debug_package %{nil}