[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

llvm-svn: 245833