Fix kernel build ${MACHINE} path

$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

sys/conf/Makefile.pc98

@@ -28,6 +28,9 @@ S=	./@
 S=	../../..
 .endif
 .endif
+
+LDSCRIPT_NAME?= ldscript.${MACHINE_ARCH}
+
 .include "$S/conf/kern.pre.mk"
 
 ASM_CFLAGS.mpboot.s=		${CLANG_NO_IAS34}

sys/conf/kern.pre.mk

@@ -26,7 +26,7 @@ KODIR?=		/boot/${KERNEL}
 LDSCRIPT_NAME?=	ldscript.$M
 LDSCRIPT?=	$S/conf/${LDSCRIPT_NAME}
 
-M=		${MACHINE_CPUARCH}
+M=		${MACHINE}
 
 AWK?=		awk
 CP?=		cp

sys/conf/kmod.mk

@@ -425,10 +425,10 @@ genassym.o: opt_global.h
 .endif
 assym.s: ${SYSDIR}/kern/genassym.sh
 	sh ${SYSDIR}/kern/genassym.sh genassym.o > ${.TARGET}
-genassym.o: ${SYSDIR}/${MACHINE_CPUARCH}/${MACHINE_CPUARCH}/genassym.c
+genassym.o: ${SYSDIR}/${MACHINE}/${MACHINE}/genassym.c
 genassym.o: ${SRCS:Mopt_*.h}
 	${CC} -c ${CFLAGS:N-fno-common} \
-	    ${SYSDIR}/${MACHINE_CPUARCH}/${MACHINE_CPUARCH}/genassym.c
+	    ${SYSDIR}/${MACHINE}/${MACHINE}/genassym.c
 .endif
 
 lint: ${SRCS}

sys/pc98/pc98/genassym.c

@@ -0,0 +1,3 @@
+/* $FreeBSD$ */
+
+#include "../../i386/i386/genassym.c"