powerpc: Split exception handling out of head_64.S

To prepare for future support of Book3E 64-bit PowerPC processors,
which use a completely different exception handling, we move that
code to a new exceptions-64s.S file.

This file is #included from head_64.S due to some of the absolute
address requirements which can currently only be fulfilled from
within that file.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>