make vp8_recon16x16mb{,y} RTCD functions

ARM NEON has a platform specific version of vp8_recon16x16mb, though
it's just a stub to extract the various parameters from the
MACROBLOCKD struct and pass them to vp8_recon16x16mb_neon(). Using
that function's prototype directly will be a better long term solution,
but it's quite an invasive change.

Change-Id: I04273149e2ade34749e2d09e7edb0c396e1dd620