arch: xtensa: add workaround for small vector table entries

For some platforms, like NXP's IMX8 or Mediatek's MT8195,
the size of an interrupt vector table entry is 0x1C bytes,
less than usual (0x30 for Intel's platforms).
So, the interrupt handlers don't fit in the vector table
entries.

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.
For this, I've added a generic configuration -
XTENSA_SMALL_VECTOR_TABLE_ENTRY.

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