arm: add connecting exceptions at runtime

Add sys_exc_connect() (and its x86-compatible alias nanoCpuExcConnect())
that allows connecting an exception handler at runtime.

The current implementation is a bit of a bastard, to avoid disturbing
the current implementation of the exception handlers. Instead of hooking
_exc_wrapper() in all vectors and adapting the exception handlers, the
current exception handlers are still hooked directly in the vectors.
When an exception is hooked at runtime, _exc_wrapper() gets installed in
the vector and the real handler gets inserted in _sw_exc_table; this
means that the scheme only works with non-XIP kernels.

This should be enhanced so that _exc_wrapper() is hooked in all vectors,
and that current exception handlers (for faults mostly) are reworked to
be inserted in the _sw_exc_table and wrapped in _exc_wrapper().

Change-Id: Icaa14f4835b57873d2905b7fbcbb94eeb3b247d1
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>