Assemble psych and include it in the build. (#232)

Psych was added to the build system. It requires fops.s
for maths. Two notable things need to be fixed to make it
assemble:

 * d1 must be defined, presumably to a 1 following the
   patterns of other decimal numbers.

 * variable xx must be renamed so it doesn't clash with
   the xx instruction mnemonic used by the perl cross assembler.

build/Makefile

@@ -133,6 +133,7 @@ cmd:	dirs \
 	$(BINDIR)/moo \
 	$(BINDIR)/nm \
 	$(BINDIR)/p \
+	$(BINDIR)/psych \
 	$(BINDIR)/rm \
 	$(BINDIR)/rn \
 	$(BINDIR)/roff \
@@ -227,6 +228,10 @@ P=$(CMDSRC)/p1.s $(CMDSRC)/p2.s $(CMDSRC)/p3.s $(CMDSRC)/p4.s $(CMDSRC)/p5.s
 $(BINDIR)/p: $(P)
 	$(AS) $(ASARGS) -o $(BINDIR)/p $(P)
 
+PSYCH=$(CMDSRC)/psych.s $(CMDSRC)/fop.s
+$(BINDIR)/psych: $(PSYCH)
+	$(AS) $(ASARGS) -o $(BINDIR)/psych $(PSYCH)
+
 $(BINDIR)/rm: $(CMDSRC)/rm.s
 	$(AS) $(ASARGS) -o $(BINDIR)/rm $(CMDSRC)/rm.s