Writes cpu variant information to dev/

This change adds a command to init.rc to write cpu variant information to a file under dev/

Test: sync to device and make sure corresponding files are created.
Change-Id: Ibf90967f13f72af925c82ff79bd973ef4cdc4068

Former-commit-id: d60a19b184e7f337d8a695c9c915afff935c04ba

rootdir/init.rc

@@ -269,6 +269,12 @@ on init
     # that they can be chown'd to system:system later on boot
     write /sys/class/leds/vibrator/trigger "transient"
 
+    # This is used by Bionic to select optimized routines.
+    write /dev/cpu_variant:${ro.bionic.arch} ${ro.bionic.cpu_variant}
+    chmod 0444 /dev/cpu_variant:${ro.bionic.arch}
+    write /dev/cpu_variant:${ro.bionic.2nd_arch} ${ro.bionic.2nd_cpu_variant}
+    chmod 0444 /dev/cpu_variant:${ro.bionic.2nd_arch}
+
     # Setup APEX mount point and its security context
     mount tmpfs tmpfs /apex nodev noexec nosuid
     chmod 0755 /apex