Hi,

This patch fixes the case where the diagnostic on attempting to assemble 
files containing ARM instructions on processors which don't support them 
isn't very helpful (e.g. "instruction not supported -- `nop'"). Thumb 
mode is made the default on such cores, and attempts to use the ".arm" 
directive or ARM instructions is faulted.

Tested with "make check" with cross to arm-none-eabi, with a small test 
case added.

OK to apply on mainline? CSL branch?

Cheers,

Julian

ChangeLog (gas):

     * config/tc-arm.c (arm_thumb_only, arm_thumb_only_except): Add
     static vars.
     (md_assemble): Disable ARM instructions on non-ARM-supporting cores.
     (autoselect_thumb_from_cpu_variant): New function. Switch on Thumb
     mode automatically based on cpu variant.
     (md_begin): Call above function.
     (s_arm_cpu, s_arm_arch): Likewise.

ChangeLog (gas/testsuite):

     * gas/arm/noarm.s: Add test for disabled ARM insns.
     * gas/arm/noarm.d: Drive test for above.
     * gas/arm/noarm.l: Expected error output.

ChangeLog (include/opcode):

     * arm.h (THUMB_ONLY): New macro. Architectures which support only
     Thumb instructions.
     (THUMB_ONLY_EXCEPT): Bits set here negate the above cases.