PsychGPUControl: Add use of 64-Bit ATIRadeonperf_Linux64 exe on 64-Bit Linux.

We have a dedicated 64-Bit executable, so we do not require installation of
32-Bit compatibility libraries on 64-Bit Linux systems just for our little
exe here, as that would be wasteful.

Add 64-Bit compiled ATIRadeonperf_Linux64 executable.

Fixes Issue #21