[Arm] Add CSDB instruction

[Arm] Add CSDB instruction

CSDB is a new instruction which Arm has defined. As it shares the
encoding space with NOP instructions, it is available from Armv3 in
Arm mode, and Armv6T2 in Thumb mode.

OK? If so, please commit on my behalf as I don't have commit rights
over here.

Thanks, James

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opcodes/

2018-01-09  James Greenhalgh  <james.greenhalgh@arm.com>

	* arm-dis.c (arm_opcodes): Add csdb.
	(thumb32_opcodes): Add csdb.

gas/

2018-01-09  James Greenhalgh  <james.greenhalgh@arm.com>

	* config/tc-arm.c (insns): Add csdb, enable for Armv3 and above
	in Arm execution state, and Armv6T2 and above in Thumb execution
	state.
	* testsuite/gas/arm/csdb.s: New.
	* testsuite/gas/arm/csdb.d: New.
	* testsuite/gas/arm/thumb2_it_bad.l: Add csdb.
	* testsuite/gas/arm/thumb2_it_bad.s: Add csdb.