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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>