xtensa: move fast exception handlers close to vectors

On XIP kernels it makes sense to have exception vectors and fast
exception handlers together (in a fast memory). In addition, with MTD
XIP support both vectors and fast exception handlers must be outside of
the FLASH.

Add section .exception.text and move fast exception handlers to it.
Put it together with vectors when vectors are outside of the .text.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>