Create the framework for adding a thumb2 backend for ARM. So far, it...

Create the framework for adding a thumb2 backend for ARM. So far, it
is exactly the same as the ARM one. The plan is that it will slowly start 
generating THUMB2 for some instructions, until eventually it generates pure thumb2.
BUG=none
TEST=none

[Erik Corry, 2009-12-08 20:15:20]

Looks good to me.  I will commit this in the morning so there is someone around
if it breaks something and we have to revert.

http://codereview.chromium.org/464070/diff/16/20
File SConstruct (right):

http://codereview.chromium.org/464070/diff/16/20#newcode667
SConstruct:667: 'values': ['arm', 'thumb', 'none'],
Can we rename the option from thumb to thumb2 and change from variant to
armvariant?  Also above where the macro is defined.  The thumb instruction set
is a different and altogether less useful one.

http://codereview.chromium.org/464070/diff/16/20#newcode669
SConstruct:669: 'help': 'generate thumb2 instructions instead of arm
instructions'
Can we add (default) at the end of this string, since otherwise it's not clear
what the default is on an ARM target.