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Support for a.out coredumps is now conditional on EXEC_AOUT, EXEC_COFF, EXEC_ECOFF, or EXEC_MACHO, or LKM. Since coredump_netbsd is the only user of cpu_coredump, make that conditional too. Lastly, add 'no options EXEC_AOUT' so the EXEC_AOUT option in std.i386 can be overridden.