Make libvpx Chromium build friendly

Add PRIVATE macro for adding private_extern directive for yasm
to hide global symbols. This is only enabled if -DCHROMIUM is used
with YASM.

Also fixed a small problem with	rtcd_defs.sh to guard TEMPORAL_DENOISING.

Change-Id: I9027fce3ebddcf20078293e4b86b396f21da7857