arch: xtensa: modify asm for interrupt sections

For IMX, for timer interrupt, the interrupt handler
was not the correct one executed and that’s because
the handlers were not at the expected address.
For IMX the size constraint of the interrupt vector
table entry is 0x1C bytes of code, less than usual.

I've added a small indirection to bypass this size
constraint and moved the default handlers to the end
of vector table, renaming them to
_Level\LVL\()VectorHelper.

Signed-off-by: Iuliana Prodan <iuliana.prodan@nxp.com>