FindLLVM: allow LLVM/Clang 3.8 and reorder the llvm-config priority.
FindLLVM: allow LLVM/Clang 3.8 and reorder the llvm-config priority.

Beignet support 3 released llvm version, now is 3.6, 3.7, 3.8. For
LLVM 3.4, 3.5, beignet may still support them, but full test don't cover
them.

Signed-off-by: Yang Rong <78759e37dc7114caed18f74e7515c5cb519dbf6d@intel.com>
Reviewed-by: Ruiling Song <286a174c1339c6eff91abc41c3a1012c95030c95@intel.com>

wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet