Restore generic_x86 sepolicy zygote.te file.

I87d0976800557d73064e2da038315b0d019d7a60 removed zygote.te from
generic/sepolicy and generic/BoardConfig.mk but also incorrectly
removed it from generic_x86/BoardConfig.mk, even though
generic_x86/sepolicy/zygote.te still exists and contains rules
needed on the x86 emulator.  Otherwise the zygote fails with
execmem denials on the x86 emulator.

(x86 emulator is also broken currently due to yaffs2 /cache yielding
unlabeled denials, but that is unrelated to this change).

Change-Id: Ie36ed4ed7ba478a377f9a0d4383d006b49bde5cc
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>