[arm] Use arm_active_target for architecture and tune

[arm] Use arm_active_target for architecture and tune

We now start to make more use of the new data structure.  This allows
us to eliminate two of the existing static variables,
arm_selected_arch and arm_selected tune.

	* arm.c (arm_selected_tune): Delete static variable.
	(arm_selected_arch): Likewise.
	(arm_configure_build_target): Declare local versions of arm_selected
	target and arm_selected_arch.  Initialize more fields in target
	data structure.
	(arm_option_override): Use arm_active_target instead of
	arm_selected_tune and arm_selected_arch.
	(asm_file_start): Use arm_active_target.

From-SVN: r243699