libseccomp: introduce BR2_PACKAGE_LIBSECCOMP_ARCH_SUPPORTS

Instead of duplicating the architecture dependency between the main
option and the Config.in comment, add a
BR2_PACKAGE_LIBSECCOMP_ARCH_SUPPORTS hidden option.

This is done in preparation to enabling libseccomp on ARM.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>