aarch64: Add A64FX machine model

aarch64: Add A64FX machine model

This patch add support for Fujitsu A64FX, as the first step of adding
A64FX machine model.

A64FX is used in FUJITSU Supercomputer PRIMEHPC FX1000,
PRIMEHPC FX700, and supercomputer Fugaku.
The official microarchitecture information of A64FX can be read at
https://github.com/fujitsu/A64FX.

2020-08-07  Qian jianhua  <qianjh@cn.fujitsu.com>

gcc/
	* config/aarch64/aarch64-cores.def (a64fx): New core.
	* config/aarch64/aarch64-tune.md: Regenerated.
	* config/aarch64/aarch64.c (a64fx_prefetch_tune, a64fx_tunings): New.
	* doc/invoke.texi: Add a64fx to the list.

(cherry picked from commit 02f21aea0679c5cac094a3f575e839d44cb57a39)