nios2: set up common linker script for XIP and non-XIP

We will require 6 variables to be defined by SOC-specific
linker script; these values in turn can be pulled from
defines in layout.h.

To help position code correctly we define two new ELF sections
for this arch, 'reset' and 'exceptions'.

Change-Id: Idffbd53895945b7d0ec0aac281e5bf7c85b4b2c2
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>