Remove useless belt and suspenders include guards in some of our headers.

For these headers:

/usr/include/machine/atomic.h
/usr/include/machine/bus_dma.h
/usr/include/machine/coredump.h
/usr/includ

Remove useless belt and suspenders include guards in some of our headers.

For these headers:

/usr/include/machine/atomic.h
/usr/include/machine/bus_dma.h
/usr/include/machine/coredump.h
/usr/include/machine/cpufunc.h
/usr/include/machine/db_machdep.h
/usr/include/machine/elf.h
/usr/include/machine/endian.h
/usr/include/machine/frame.h
/usr/include/machine/limits.h
/usr/include/machine/npx.h
/usr/include/machine/profile.h
/usr/include/machine/segments.h
/usr/include/machine/stdarg.h
/usr/include/machine/stdint.h
/usr/include/machine/trap.h
/usr/include/machine/tss.h
/usr/include/machine/ucontext.h
/usr/include/machine/vframe.h
/usr/include/machine/vm86.h

All these headers #define _CPU_... and not _MACHINE_... even though they
are in /usr/include/machine. And the headers themselves have include
guards already. So there's little point in having them around the actual
#include additionally.

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vkernel64: Do some more replacing amd64 -> x86_64.

This reduces differences with the non-vkernel 64 bit code.

vkernel64: Additional adjustments (amd64 -> x86_64, recent commits etc.).

Add platform vkernel64.