Define rump_nativeabi_p() under arch, where one would logically expect
it to be defined.

Rototill a bit, and attempt to disguise it as non-gratuitous.

Add arch/generic and move non-x86 files from rumpkern/ there. Also,
move files from arch/i386 to arch/x86, and make both i386 and x86_6

Rototill a bit, and attempt to disguise it as non-gratuitous.

Add arch/generic and move non-x86 files from rumpkern/ there. Also,
move files from arch/i386 to arch/x86, and make both i386 and x86_64
use those.

This clarifies the situation with what is MD vs. MI code.

renames:
rumpcpu_generic,kobj_stubs,pmap_stubs => arch/generic/rump_generic_$x
arch/i386/* => arch/x86/rump_x86_$x

(for those who forget, x86 requires MD code because rump kernels
use the same ABI as kernel modules)

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