Fix RPM backend does not honor package overrides for PACKAGE_ARCH

Fix RPM backend does not honor package overrides for PACKAGE_ARCH

If we use a package overrides as part of PACKAGE_ARCH when using the rpm
backend, The override is not honored.

For example, we use the Randy Witt's attachment, we can get it from:

	https://bugzilla.yoctoproject.org/show_bug.cgi?id=7984

Using the attached recipe, the package "package-b" would be have architecture
of "all" in the rpm metadata, but for the subpackages rpm program only support
"noarch" , so at this time we set it to "noarch", after we build it, however
it ends up being equal to the default. i.e. TUNE_PKGARCH, but it does not occur
when using ipk and deb.

This is because when using ipk and deb, the packages were builded one by one,
but when using rpm, the packages were builded form only one spec file, that is
to say the building only occur once, and all the packages' arch was set to the
default: TUNE_PKGARCH.

The solution is that we need to check the extra arch of packages, and build them
individually.

[YOCTO #7984]

Signed-off-by: Dengke Du <dengke.du@windriver.com>