nios2: linker.ld: keep exception code for non-XIP

The KEEP() is only necessary for the exception entry point
as it sits at a magic memory address and isn't referenced by
other code.

Change-Id: I8443e8aa23059b65eaf9c5a1cf3f9b14b04737d5
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>