Define BOARD_SEPOLICY_DIRS for 64-bit emulators.

Define BOARD_SEPOLICY_DIRS for the arm64, mips64, and x86_64
emulator targets.  As a first cut, simply inherit from the
existing policy directories used for generic and generic_x86.
We may need further board-specific policy added for these targets
but testing will require first enabling SELinux in the relevant
kernel configs.

(cherrypicked from commit 21ebc213)

Change-Id: I7b4459b32298698fc2908cbbdd0e3afadbe5ac24
Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>