Enable SSSE3 implementation of 8x8 forward 2D-DCT

Assembly implementation of ssse3 8x8 forward 2D-DCT. The current
version is turned on only for x86_64. The average unit runtime
goes from 157 cycles down to 136 cycles, i.e., about 12.8% faster.
This translates into about 1.5% speed-up for pedestrian_area 1080p
at speed 2.

Change-Id: I0f12435857e9425ed7ce12541344dfa16837f4f4