Fix and improve exception tracing:
1.  Name the kernel option XTRACE instead of EXCEPTION_TRACING
2.  Put support functions in ia64/ia64/xtrace.c
3.  Make it work with SMP by giving each CPU its own buffer
4.  Save 16 key registers in the buffer for every exception
5.  In ia64_handle_intr() and trap() transfer the trace record
    to the KTR trace buffer using CTRx() and with some basic
    information for now
6.  Use a tunable to anble tracing and stop tracing as soon as
    we enter the debugger

Room for improvements:
1.  Transferring exception-relevant information to KTR
2.  Add a sysctl to enable/disable tracing