Fix half-pixel variance RTCD functions

Fix half-pixel variance RTCD functions

This patch fixes the system dependent entries for the half-pixel
variance functions in both the RTCD and non-RTCD cases:

  - The generic C versions of these functions are now correct.
    Before all three cases called the hv code.

  - Wire up the ARM functions in RTCD mode

  - Created stubs for x86 to call the optimized subpixel functions
    with the correct parameters, rather than falling back to C
    code.

Change-Id: I1d937d074d929e0eb93aacb1232cc5e0ad1c6184