Add GpuRegionTimer class

Add GpuRegionTimer class

GpuRegionTimer class is implemented in CUDA/OpenCL to help timing
regions of GPU code. For CUDA, it contains pair of CUDA start/stop events;
for OpenCL, it contains fixed-size array of OpenCL events, which are
passed manually into OpenCL calls within region.
Using this class, cu/cl_timers_t structures and the GPU timing code
are made a bit more uniform.

Change-Id: I8fcd7568653c5bcf3496a2bee227ae03f14a756a