[AArch64] Support gnu vector in inferior call

[AArch64] Support gnu vector in inferior call

As defined in AArch64 AAPCS, short vectors are passed through V
registers, and its maximum alignment is 16-byte.  This patch is
to reflect these rules in GDB.  This patch fixes some fails in
gdb.base/gnu_vector.exp.

gdb:

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

	* aarch64-tdep.c (aarch64_type_align): For vector type, return
	its length, but with the maximum of 16 bytes.
	(is_hfa): Return zero for vector type.
	(aarch64_push_dummy_call): Handle short vectors.
	(aarch64_extract_return_value): Likewise.
	(aarch64_store_return_value): Likewise.