ARM: Allow __fp16 as a function arg or return type for AArch64

ACLE 2.0 allows __fp16 to be used as a function argument or return
type. This enables this for AArch64.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212812 91177308-0d34-0410-b5e6-96231b3b80d8