I added ro.bionic.(2nd_)?_(arch|cpu_variant) to vendor system
properties. And have init to write them to files under dev/.

This change set SELinux rules for these properties and files.

For the system properties: vendor/default.prop will set them. init will
read them.
For the files /dev/cpu_variant:.*: init will write them. bionic libc
will read them. (Basically world readable).

This is to allow libc select the right optimized routine at runtime.
Like memcpy / strcmp etc.

Test: getprop to make sure the properties are set.
Test: ls -laZ to make sure /dev/cpu_variant:.* are correctly labeled.

Change-Id: I41662493dce30eae6d41bf0985709045c44247d3