Using "call" on a function that passes arguments via float registers can cause
gdb to overflow buffers.

Ensure enough memory is reserved to hold a full FP register.

This fixes gdb.base/callfuncs.exp for Aarch64 SVE.

2018-09-18  Alan Hayward  <alan.hayward@arm.com>

	* aarch64-tdep.c (pass_in_v): Use register size.
	(aarch64_extract_return_value): Likewise.
	(aarch64_store_return_value): Likewise.