xtensa: Make high priority interrupts optional

Xtensa has a "high priority" class of interrupt levels which ignore
the EXCM bit and can thus interrupt running exception handlers.  These
can't be used for C handlers in the general case[1] because C code
needs to be able to throw window over/underflow exceptions, which are
not reentrant.

But the high priority interrupts might be useful to a carefully
designed application, or to unit tests of low level architecture code.
So make their generation optional with this kconfig option.

[1] ESP-32 has a high priority interrupt for its watchdog, apparently.
    Which is sort of OK given that it never needs to return to the
    interrupted code.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>