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

v2: Fix reference to Vega12/20 enabling commit

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewer: Aaron Watry
llvm-svn: 342341
GitOrigin-RevId: 70c5f9dff8ae6d4b385974074145cfcf9e48821d
1 file changed