[AArch64] Handle HFA and HVA together

[AArch64] Handle HFA and HVA together

AArch64 AAPCS defined HFA (homogeneous floating-point aggregate)
and HVF (homogeneous short vector aggregate), bug GDB only handles the
former.  In the AAPCS doc, both types are treated exactly the same
in terms of alignment and passing locations (on registers or stack).
This patch is to extend is_hfa to handle both HFA and HVA.

gdb:

2015-11-27  Yao Qi  <yao.qi@linaro.org>

	* aarch64-tdep.c (is_hfa): Rename to ...
	(is_hfa_or_hva): ... this.  Handle vector type.  All callers
	updated.
	(aarch64_extract_return_value): Update debugging message.
	(aarch64_store_return_value): Likewise.
	(aarch64_return_in_memory): Update comments.