[PATCH] configure: Rework support for gfx9+ devices that were added post LLVM 3.9

Runtimes OpenCL
#1

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

#2

LGTM, although you might want to check that commit ID for the LLVM 7
addition of vega12/20. I suspect there's one too many 3's in that.

--Aaron