A fix for the clang + eabi + kdb backtrace endless loop

[Ian Lepore]

The attached patch fixes the problem where a kdb backtrace loops
endlessly on an eabi kernel.  

I'm no expert on this stuff, so while this fixes the problem for me, I'm
not sure it's right, especially the STOP_UNWINDING in exception_exit
(which handles a test case of breaking into the debugger with the
keyboard and typing 'bt').  I'm not going to commit this until it's been
reviewed by someone who actually knows what they're doing. :)

-- Ian

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