Use onbld mandoc.

@@ -944,10 +944,12 @@
 #
 # This is only used during the -t tools build
 #
 opt/onbld/bin/i386/elfsign      i386
 opt/onbld/bin/sparc/elfsign     sparc
+opt/onbld/bin/i386/mandoc       i386
+opt/onbld/bin/sparc/mandoc      sparc
 
 #
 # Private libdwarf
 #
 opt/onbld/lib/i386/libdwarf.so  i386