Add length, normalize and their native variants for f16

http://b/23536224

https://android-review.googlesource.com/#/c/186326/ included
implementations for length, native_length, normalize and
native_normalize in driver/runtime.  I had missed adding them to the
API.  This CL adds specs for these functions.

Change-Id: Idaefb7ec2e45adb7b0e4b502404b1098f4c9d8c9
1 file changed