attempt to fix -cl-ext with LLVM 4.0
attempt to fix -cl-ext with LLVM 4.0

The previous code was incorrectly producing multiple -cl-ext
options, instead of a single option with +/- feature list.

franz/pocl,Oblomov/pocl,Oblomov/pocl,rjodin/pocl,franz/pocl,rjodin/pocl,pocl/pocl,pocl/pocl,Oblomov/pocl,franz/pocl,franz/pocl,pocl/pocl,Oblomov/pocl,pocl/pocl,pocl/pocl,Oblomov/pocl,franz/pocl,rjodin/pocl,rjodin/pocl