target.cfg: remove "-work-area-virt 0"

The semantics of "-work-area-virt 0" (or phys) changed with
the patch to require specifying physical or virtrual work
area addresses.  Specifying zero was previously a NOP.  Now
it means that address zero is valid.

This patch addresses three related issues:

 - MMU-less processors should never specify work-area-virt;
   remove those specifications.  Such processors include
   ARM7TDMI, Cortex-M3, and ARM966.

 - MMU-equipped processors *can* specify work-area-virt...
   but zero won't be appropriate, except in mischievous
   contexts (which hide null pointer exceptions).

   Remove those specs from those processors too.  If any of
   those mappings is valid, someone will need to submit a
   patch adding it ... along with a comment saying what OS
   provides the mapping, and in which context.  Example,
   say "works with Linux 2.6.30+, in kernel mode".  (Note
   that ARM Linux doesn't map kernel memory to zero ...)

 - Clarify docs on that "-virt" and other work area stuff.

Seems to me work-area-virt is quite problematic; not every
operating system provides such static mappings; if they do,
they're not in every MMU context...

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>

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