target-ppc: Add support for new NMI interface

This implements an NMI interface POWERPC SPAPR machine.
This enables an "nmi" HMP/QMP command supported on SPAPR.

This calls POWERPC_EXCP_RESET (vector 0x100) in the guest to deliver NMI
to every CPU. The expected result is XMON (in-kernel debugger) invocation.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
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Changes:
v6:
* support NMI interface

v4:
* s/\<nmi\>/nmi_monitor_handler/
* added note about XMON into commit log

v3:
* ppc_cpu_do_nmi() is exported from excp_helper.c instead of powerpc_excp()