[Arm] Set Armv7-A tune to Cortex-A53

[Arm] Set Armv7-A tune to Cortex-A53

By default Armv7-A tunes for Cortex-A8. This is an ancient core
today and the settings are no longer useful for newer cores. So
switch to Cortex-A53 tuning since it works well across a wide range
of modern cores.

On SPECINT2006 the performance gain is 0.7% compared to Cortex-A8 tuning,
and codesize reduces by 0.2%.

    gcc/
	* config/arm/arm-cpus.in (armv7): Set tune to Cortex-A53.
	(armv7-a): Likewise.
	(armv7ve): Likewise.

From-SVN: r278447