Follow-up to r307866:

• Make !KDB config buildable.
• Simplify interface to nmi_handle_intr() by evaluating panic_on_nmi in one place, namely nmi_call_kdb(). This allows to remove do_panic argument from the functions, and to remove i386/amd64 duplication of the variable and sysctl definitions. Note that now NMI causes panic(9) instead of trap_fatal() reporting and then panic(9), consistently for NMIs delivered while CPU operated in ring 0 and 3.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks