$M should be the kernel machine src directory, ${MACHINE}. In most cases
${MACHINE} and ${MACHINE_CPUARCH} are the same, but this is not true for
pc98 and arm64.
It appears we previously set M=${MACHINE_CPUARCH} as a workaround to
accommodate pc98, where MACHINE_CPUARCH is pc98 but it uses
sys/i386/i386/genassym.c.
arm64 relies on this being set correctly, so update $M and add explicit
workarounds for pc98.
Differential Revision: https://reviews.freebsd.org/D2307
Reviewed by: andrew, imp
Sponsored by: The FreeBSD Foundation
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