Formalize the use of linker scripts for kernel modules.

Automatically apply ldscript.kmod.${MACHINE_ARCH} if it exists.
We already have an i386-specific linker script; rename it accordingly.

Note that the linker script is applied when the object files are
partially linked.  (For amd64 this is also the final link.)

Reviewed by:	imp, kib
Discussed with:	jhb
MFC after:	1 week
Sponsored by:	Netflix
Differential Revision:	https://reviews.freebsd.org/D21887

sys/conf/kmod.mk

@@ -242,18 +242,16 @@ EXPORT_SYMS?=	NO
 CLEANFILES+=	export_syms
 .endif
 
+.if !defined(FIRMWS) && exists(${SYSDIR}/conf/ldscript.kmod.${MACHINE_ARCH})
+_LDFLAGS+=-T ${SYSDIR}/conf/ldscript.kmod.${MACHINE_ARCH}
+.endif
+
 .if ${__KLD_SHARED} == yes
 ${KMOD}.kld: ${OBJS}
 .else
 ${FULLPROG}: ${OBJS}
 .endif
-.if !defined(FIRMWS) && (${MACHINE_CPUARCH} == "i386")
-	${LD} -m ${LD_EMULATION} ${_LDFLAGS} -r \
-	    -T ${SYSDIR}/conf/ldscript.set_padding \
-	    -d -o ${.TARGET} ${OBJS}
-.else
 	${LD} -m ${LD_EMULATION} ${_LDFLAGS} -r -d -o ${.TARGET} ${OBJS}
-.endif
 .if ${MK_CTF} != "no"
 	${CTFMERGE} ${CTFFLAGS} -o ${.TARGET} ${OBJS}
 .endif