stand/kboot: Add note about why we use MACHINE_ARCH here

Normally in the boot loader, we key off of MACHINE since that specifies
the kernel and the loader is very tuned to each type of MACHINE in
general. In this case, however, we're producing a Linux binary, with
Linux system calls encoded in it. These align better along the
MACHINE_ARCH axis of FreeBSD. For PowerPC the system calls are radically
different for each of our MACHINE_ARCHes, with only powerpc64 and
powerpc64le sharing the same numbers and memory layout. The same was
true about mips when it was in the tree. 32-bit arm uses the same
layout, however, for both armv6 and armv7 ports: that can be easily
shared in the unlikely event we support that in the future.

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