arc: early init should invalidate d-cache and set vector table

Some ARC targets can have a data-cache. Although there is no special
instruction to clear exceptions during early init, it is necessary to
invalidate the d-cache BEFORE any data is fetched. The ARC on arduino 101
doesn't have d-cache, and will thus skip this d-cache invalidate.

Also, it is important to set the vector table base register to point to
the interrupt vector table EARLY, so that if an exception is encountered,
the correct vector table is found. Set this base only if it is found to be
different from the one compiled in to the code.
These initialization steps assure that proper exception handling
is in place during early init.

Change-Id: Ie8b5928e5813e104680a6d6510c85d32dc8ed8f3
Signed-off-by: Chuck Jordan <cjordan@synopsys.com>