also allow building .s files from .c ones

... along the lines of allowing .i files to be built from .c ones as
well as .s from .S (aiding the analysis of occasional build problems).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/Makefile
+++ b/xen/Makefile
@@ -195,7 +195,7 @@ _MAP:
 .PHONY: FORCE
 FORCE:
 
-%.o %.i: %.c FORCE
+%.o %.i %.s: %.c FORCE
 	$(MAKE) -f $(BASEDIR)/Rules.mk -C $(*D) $(@F)
 
 %.o %.s: %.S FORCE
--- a/xen/Rules.mk
+++ b/xen/Rules.mk
@@ -177,6 +177,9 @@ $(filter %.init.o,$(obj-y) $(obj-bin-y) 
 %.i: %.c Makefile
 	$(CPP) $(CFLAGS) $< -o $@
 
+%.s: %.c Makefile
+	$(CC) $(CFLAGS) -S $< -o $@
+
 # -std=gnu{89,99} gets confused by # as an end-of-line comment marker
 %.s: %.S Makefile
 	$(CPP) $(AFLAGS) $< -o $@