arm: Ignore --with-mode when CPU only supports one instruction set.

arm: Ignore --with-mode when CPU only supports one instruction set.

Arm processors can support up to two instruction sets.  Some early
cores only support the traditional A32 (Arm) instructions, while some
more recent devices only support T32 (Thumb) instructions.

When configuring the compiler,--with-mode can be used to select the
default instruction set to target if the user has not made an explicit
choice, but this can cause needless problems if the default is not
supported by the requested CPU.

To fix this this patch adjusts the way that the --with-mode selection
is processed so that it can take into account the selected CPU or
architecture and not create a meaningless combination.

gcc:
	* common/config/arm/arm-common.c: Include configargs.h.
	(arm_config_default): New function.
	(arm_target_mode): Renamed from arm_target_thumb_only.  Handle
	processors that do not support Thumb.  Take into account the
	--with-mode configuration setting for selecting the default.
	* config/arm/arm.h (OPTION_DEFAULT_SPECS): Remove entry for 'mode'.
	(TARGET_MODE_SPEC_FUNCTIONS): Update for function name change.