Consolidate Thumb-1/Thumb-2 ISA detection

Consolidate Thumb-1/Thumb-2 ISA detection

2015-12-24  Thomas Preud'homme  <thomas.preudhomme@arm.com>

gas/
    * config/tc-arm.c (move_or_literal_pool): Check mov.w, mvm and movw
    availability against arm_ext_v6t2 instead of checking arm_arch_t2,
    fixing comments along the way.
    (handle_it_state): Check arm_ext_v6t2 instead of arm_arch_t2 to
    generate IT instruction.
    (t1_isa_t32_only_insn): New function.
    (md_assemble): Use above new function to check for invalid wide
    instruction for CPU Thumb ISA and to determine what Thumb extension
    bit is necessary for that instruction.
    (md_apply_fix): Use arm_ext_v6t2 instead of arm_arch_t2 to decide if
    branch is out of range.

include/opcode/
    * arm.h (ARM_ARCH_THUMB2): Add comment explaining its meaning and
    remove extension bit not including any Thumb-2 instruction.