[arm] Rework arm-common to use new feature bits.

[arm] Rework arm-common to use new feature bits.

This converts the recently added implicit -mthumb support code to use
the new data structures.  Since we have a very simple query and no
initialized copies of the sbitmaps, for now we simply scan the list of
features to look for the one of interest.

	* arm-opts.h (struct arm_arch_core_flag): Add new field ISA.
	Initialize it.
   	(arm_arch_core_flag): Delete flags field.
    	(arm_arch_core_flags): Don't initialize flags field.
 	* common/config/arm/arm-common.c (check_isa_bits_for): New function.
	(arm_target_thumb_only): Use new isa bits arrays.

From-SVN: r243704