Correction to -mfu=neon-vfpv4 to pass the correct backend feature name

Change-Id: I4dbfe1d97670fc4e626368ef1f91fc008778dfca

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224523 91177308-0d34-0410-b5e6-96231b3b80d8