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

git-svn-id: e7f1ab7c2a259259587475a3e7520e757882f167@342341 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libclc,llvm-mirror/libclc,llvm-mirror/libclc,llvm-mirror/libclc,llvm-mirror/libclc,llvm-mirror/libclc,llvm-mirror/libclc