kbuild: rpm-pkg: record ARCH option in spec file

Currently, we rely on the top Makefile defining ARCH option when we
run 'make rpm-pkg' or 'make binrpm-pkg'.

It does not apply when we run 'make srcrpm-pkg', and separately run
'rpmbuild' for the generated SRPM. This is a problem for cross-build.

Just like the Debian package, save the value of ARCH in the spec file.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>