Arm: Fix disassembler crashing on -b binary when thumb file and thumb not forced.

Arm: Fix disassembler crashing on -b binary when thumb file and thumb not forced.

The disassembler for Arm has some aborts in it in places it assumes can never
be reached.  Under normal circumstances they indeed cannot be reached because
the right options are selected from the ARM attributes in the ELF file.

However when disassembling with -b binary then if you do not get the options
right the disassembler just aborts.  This changes it so it just prints how it
was trying to interpret the instruction and prints UNKNOWN instructions next to it.

This way the user has an idea of what's going.

gas/ChangeLog:

	* testsuite/gas/arm/undefined-insn-arm.d: New test.
	* testsuite/gas/arm/undefined-insn-thumb.d: New test.
	* testsuite/gas/arm/undefined-insn.s: New test.

opcodes/ChangeLog:

	* arm-dis.c (UNKNOWN_INSTRUCTION_32BIT): Format specifier for arm mode.
	(UNKNOWN_INSTRUCTION_16BIT): Format specifier for thumb mode.
	(print_insn_arm, print_insn_thumb16, print_insn_thumb32): Use them.