[AArch64] Fix erroneous use of spu architecture bfd

[AArch64] Fix erroneous use of spu architecture bfd

While investigating some SVE code, i noticed the use of two spu bfd variables.

This looks like an oversight, as the "id" field is available for non-spu
architectures as well, even though its primary use was the Cell BE
architecture.

gdb/ChangeLog:

2020-01-05  Luis Machado  <luis.machado@linaro.org>

	* aarch64-linux-nat.c
	(aarch64_linux_nat_target::thread_architecture): Use bfd_arch_aarch64
	and bfd_mach_aarch64.