arm: Add option for mitigating against Cortex-A CPU erratum for AES

arm: Add option for mitigating against Cortex-A CPU erratum for AES

Add a new option -mfix-cortex-a-aes for enabling the Cortex-A AES
erratum work-around and enable it automatically for the affected
products (Cortex-A57 and Cortex-A72).

gcc/ChangeLog:

	* config/arm/arm-cpus.in (quirk_aes_1742098): New quirk feature
	(ALL_QUIRKS): Add it.
	(cortex-a57, cortex-a72): Enable it.
	(cortex-a57.cortex-a53, cortex-a72.cortex-a53): Likewise.
	* config/arm/arm.opt (mfix-cortex-a57-aes-1742098): New command-line
	option.
	(mfix-cortex-a72-aes-1655431): New option alias.
	* config/arm/arm.cc (arm_option_override): Handle default settings
	for AES erratum switch.
	* doc/invoke.texi (Arm Options): Document new options.