radeon_noop backend

This implements the minimum of the radeon kernel driver in order to make shader-db work. The submit ioctl is stubbed out to not execute anything.

Export MESA_LOADER_DRIVER_OVERRIDE=r300 LD_PRELOAD=$prefix/lib/libradeon_noop_drm_shim.so. (or r600 for r600-class HW)

By default, rv515 is exposed. The chip can be selected an enviornment variable like RADEON_GPU_ID=CAYMAN or RADEON_GPU_ID=0x6740.