Remove TEXTREL from 32bit encoder

This patch fixed the issue reported in "Issue 655: remove textrel's
from 32-bit vp9 encoder". The set of vp9_subpel_variance functions
that used x86inc.asm ABI didn't build correctly for 32bit PIC. The
fix was carefully done under the situation that there was not
enough registers.

After the change, we got
$ eu-findtextrel libvpx.so
eu-findtextrel: no text relocations reported in 'libvpx.so'

Change-Id: I1b176311dedaf48eaee0a1e777588043c97cea82