versions for thumb and thumb-2 instruction sets

[Dan Olson via D.gnu]

Hi Iain, Johannes.  I'm asking here because I don't expect anybody else
is playing with thumb instructions.

Both GDC and LDC have version ARM_Thumb defined for original thumb (call
it thumb-1) and thumb-2.  But there is no way to tell in D code which
set is supported.

GCC and clang define these:

__arm__
Always defined when building for the ARM architecture (regardless of
target instruction set)

__thumb__
Defined only when targeting any Thumb instruction set variant (Thumb-1
or Thumb-2)

__thumb2__
Defined only when targeting Thumb-2

We could mimic this by adding ARM_Thumb2.

But in my experience it is usually thumb-1 that needs special handling
because it is very limited, and thumb-2 uses unified assembler so often
same code works for either thumb-2 or arm modes.

So I am thinking that ARM_Thumb1 would be more practical.  What do you
think?
-- 
Dan