Coalesce generic_x86 and generic sepolicy where possible.

We originally forked a complete copy of generic/sepolicy into
generic_x86/sepolicy, but we can instead inherit from it and
merely add rules as needed under generic_x86/sepolicy.

Change-Id: I21e1a1425ce08676a8ea69685a4761db3bfde628
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>