Implement the assembly instructions yield, wfe, wfi and sev for ARMv6T2 in...

Implement the assembly instructions yield, wfe, wfi and sev for ARMv6T2 in both ARM mode and Thumb mode.

	* config/tc-arm.c (arm_ext_v6k_v6t2): Define.
	(insns) [ARM_VARIANT]: Modified.
	(insns) [THUMB_VARIANT]: To implement few ARMv6K instructions
	in ARMv6T2 as well.
	* testsuite/gas/arm/archv6t2-1.d: New test.
	* testsuite/gas/arm/archv6t2-1.s: Likewise.
	* testsuite/gas/arm/archv6t2-2.d: Likewise.