[PATCH v2 1/2] Introduce per device defines

Make cl_khr_fp64 define per-device.
This patch does not change the generated Makefile

v2: Make the device defines per LLVM version, 'all' for all versions

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>

Also remove definitions if provided by clang (3.7+)
This halves the size of {cedar,barts}-r600--.bc
All other binaries are identical.

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>