[ARM] Add new macro for Thumb-only opcodes

[ARM] Add new macro for Thumb-only opcodes

Armv8-M Security Extensions introduced some Thumb-only opcodes
(eg. sg). These are defined using the TUE and TCE macros, setting the
Arm execution state related fields to 0/NULL.

This patch adds 2 new macros to avoid filling this field and clearly
identify Thumb-only instructions.

2018-01-15  Thomas Preud'homme  <thomas.preudhomme@arm.com>

gas/
	* config/tc-arm.c (ToC): Define macro.
	(ToU): Likewise.
	(insns): Make use of above macros for new instructions introduced in
	Armv8-M.