systemd: enable build for supported architectures.

Systemd only supports the architectures defined in
src/shared/architecture.h. So via a new parameter named
BR2_PACKAGE_SYSTEMD_ARCH_SUPPORTS, we enable the build of this package
only for those architectures.

Fixes http://autobuild.buildroot.net/results/bcc/bcce95eb0748505c0d62db21a6e420aae3c3181b/



Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>