Move the exception vector table (so-called "page0" data) into exception.S
and eliminate vectors.S.  All low-level exception handling is now
consolidated into exception.S.

Along with moving the default FIQ handler, change it to disable FIQs
before returning.  An FIQ should never happen, but if it does, it's got
to be disabled as part of ignoring it.

In general, we have hand-wavy support for FIQs that probably hasn't been
used for 10 years and probably doesn't work (almost certainly doesn't
work for SMP because it only updates the vector on the current cpu).  This
change doesn't really make the overall situation any better or worse.