[AArch64] Define the macro __ARM_FP16_ARGS

[AArch64] Define the macro __ARM_FP16_ARGS

The ACLE (ARM C Language Extensions) 2.0 defines that the predefined macro
__ARM_FP16_ARGS should be defined if __fp16 can be used as an argument and
result.

The support for __fp16 to be used as an argument and result is already
implemented for AArch64 so this change is just adding the missing macro.

Differential Revision: http://reviews.llvm.org/D12240

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@245833 91177308-0d34-0410-b5e6-96231b3b80d8