[arm] Use strings for -march, -mcpu and -mtune options

In order to support more complex specifications for cpus and architectures
we need to move away from using enumerations to represent the set of
permitted options.  This basic change just moves the option parsing
infrastructure over to that, but changes nothing more beyond generating
a hint when the specified option does not match a known target (previously
the help option was able to print out all the permitted values, but we
can no-longer do that.

	* config/arm/arm.opt (x_arm_arch_string): New TargetSave option.
	(x_arm_cpu_string, x_arm_tune_string): Likewise.
	(march, mcpu, mtune): Convert to string-based options.
	* config/arm/arm.c (arm_print_hint_for_core_or_arch): New function.
	(arm_parse_arch_cpu_name): New function.
	(arm_configure_build_target): Use arm_parse_arch_cpu_name to
	identify selected architecture or CPU.
	(arm_option_save): New function.
	(TARGET_OPTION_SAVE): Redefine.
	(arm_option_restore): Restore string options.
	(arm_option_print): Print string options.

From-SVN: r249279