AArch64: add support for DFP (Decimal Floating point)

This small patch adds support for TYPE_CODE_DECFLOAT in
aapcs_is_vfp_call_or_return_candidate_1 and pass_in_v_vfp_candidate,
so that GDB for AArch64 knows how to pass DFP parameters and how to
read DFP results when calling a function.

Tested on aarch64-linux-gnu, with a GCC with DFP support in the PATH,
all of GDB's DFP tests pass.