[ARM] Complete enumeration values for Tag_ABI_VFP_args

The LLD implementation of Tag_ABI_VFP_args needs to check the rarely seen
values of 3 (toolchain specific) and 4 compatible with both Base and VFP.
Add the missing enumeration values so that LLD can refer to them without
having to use the raw numbers.

Differential Revision: https://reviews.llvm.org/D50049

llvm-svn: 338373