IIUC, the normal %ifarch syntax cannot be used, because the definition
needs to be inline. The best I could find is %_target_cpu, which seems to be
set to "noarch" for noarch builds.
When subpackages are used, and have different Version fields, we end up with
the version of the last subpackage in %version. But RPM_PACKAGE_VERSION is
set early, and seems to have the right version.
Fixes rhbz#2043143.
The idea was that we can avoid unnecessary work if the macro is called more than
once. But in hindsight this might be risky: let's instead minimize the number of
places where the macros is called, but always overwrite the file so that we
don't end up with a stale version from a previous build.
