xtensa: implement early_trap_init

Paging on xtensa architecture requires functioning exception handling
because hardware cannot transparently access page tables that are not
currently mapped by TLB. Exception handling is set up late in the
initialization process, but working paging is needed for KASAN.

Provide early_trap_init that sets up minimal exception handling
sufficient for KASAN to work.

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