exceptions: use a separate exception stack

Exceptions are challenging on x64 because the ABI reserves 128 bytes below
%rsp for local variables; this will be overwritten by an exception or
interrupt.  We can't use -mno-red-zone because the payload may not have been
compiled for it.  Instead, use the interrupt stack stack table mechanism
and a separate stack for exceptions.  May fix random corruptions when timers
are used.