Parallel builds race: the library target writes into ../build/$(PORT) but does
not depend on it, so with -j the archiver can run before the directory exists:

  sdar: ../build/small/mcs51.lib: No such file or directory

Give the target an order-only dependency on its output directory.

Index: device/lib/mcs51/Makefile.in
===================================================================
--- device/lib/mcs51/Makefile.in	(revision 17011)
+++ device/lib/mcs51/Makefile.in	(working copy)
@@ -30,7 +30,10 @@
 
 all: $(PORTDIR)/$(LIB)
 
-$(PORTDIR)/$(LIB): $(OBJ) Makefile
+$(PORTDIR):
+	mkdir -p $@
+
+$(PORTDIR)/$(LIB): $(OBJ) Makefile | $(PORTDIR)
 ifeq ($(LIB_TYPE), SDCCLIB)
 	rm -f $@; \
 	$(top_builddir)/bin/sdcclib -a $@ $(OBJ)