nativesdk.bbclass: Set PACKAGE_ARCHS to SDK_PACKAGE_ARCHS.
This patch fixed the issue of reading incorrect pkgdata files. Previous
appending '-nativesdk' suffix to PACKAGE_ARCHS would result the
i686-nativesdk recipes reading in x86_64-nativesdk pkgdata files if the
MACHINE is set to qemux86-64.
Fixed bug [YOCTO #2203].
Signed-off-by: Lianhao Lu <lianhao.lu@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>