[arm] Reduce usage of arm_selected_cpu.

[arm] Reduce usage of arm_selected_cpu.

Make more use of the new data structure for initializing existing
variables.

	* arm.c (arm_option_override): Use arm_active_target as source of
	information for arm_base_arch and arm_arch_name.
	* (arm_file_start): Use arm_active_target for core name.

From-SVN: r243700