kbuild: rpm-pkg: use a dummy string for _arch when undefined

If this affects only %{buildroot}, it should be enough to use a fixed
string for _arch when it is undefined.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

scripts/package/mkspec

@@ -36,6 +36,8 @@ sed -e '/^DEL/d' -e 's/^\t*//' <<EOF
 %define ARCH ${ARCH}
 %define KERNELRELEASE ${KERNELRELEASE}
 
+# _arch is undefined if /usr/lib/rpm/platform/*/macros was not included.
+%{!?_arch: %define _arch dummy}
 %{!?make: %define make make}
 %define makeflags %{?_smp_mflags} ARCH=%{ARCH}
 
@@ -55,9 +57,6 @@ $S	BuildRequires: bc binutils bison dwarves
 $S	BuildRequires: (elfutils-libelf-devel or libelf-devel) flex
 $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.
-	%{!?_arch: %define _arch $UTS_MACHINE}
 	%define __spec_install_post /usr/lib/rpm/brp-compress || :
 	%define debug_package %{nil}