Move libsysdecode-specific hack out of buildworld.

This should fix the lib32 build since it was not removing the generated
ioctl.c.  This file is generated by a find(1) call, so cannot use normal
dependency tracking methods.

Reported by:	jhb
MFC after:	2 weeks
Sponsored by:	Dell EMC Isilon

Makefile.inc1

@@ -676,9 +676,6 @@ _worldtmp: .PHONY
 .endif
 .else
 	rm -rf ${WORLDTMP}/legacy/usr/include
-#	XXX - These can depend on any header file.
-	rm -f ${OBJTREE}${.CURDIR}/lib/libsysdecode/ioctl.c
-	rm -f ${OBJTREE}${.CURDIR}/usr.bin/kdump/kdump_subr.c
 .endif
 .for _dir in \
     lib lib/casper usr legacy/bin legacy/usr

lib/libsysdecode/Makefile

@@ -114,7 +114,12 @@ DEPENDOBJS+=	tables.h
 tables.h: mktables
 	sh ${.CURDIR}/mktables ${DESTDIR}${INCLUDEDIR} ${.TARGET}
 
-ioctl.c: mkioctls
+# mkioctls runs find(1) for headers so needs to rebuild every time.  This used
+# to be a hack only done in buildworld.
+.if !defined(_SKIP_BUILD)
+ioctl.c: .PHONY
+.endif
+ioctl.c: mkioctls .META
 	env MACHINE=${MACHINE} CPP="${CPP}" \
 		/bin/sh ${.CURDIR}/mkioctls ${DESTDIR}${INCLUDEDIR} > ${.TARGET}