rework makefile subarch logic to allow shared files

instead of subarchs getting their own .s files which are used directly
by the makefile to replace the .c file, they now must provide a .sub
file whose contents are a pathname, relative to the location of the
.sub file, which will substitute for the .c file. essentially these
files are acting as symbolic links, but implemented as text files.

Makefile

@@ -90,8 +90,16 @@ $(OPTIMIZE_SRCS:%.c=%.o) $(OPTIMIZE_SRCS:%.c=%.lo): CFLAGS += -O3
 MEMOPS_SRCS = src/string/memcpy.c src/string/memmove.c src/string/memcmp.c src/string/memset.c
 $(MEMOPS_SRCS:%.c=%.o) $(MEMOPS_SRCS:%.c=%.lo): CFLAGS += $(CFLAGS_MEMOPS)
 
-%.o: $(ARCH)$(ASMSUBARCH)/%.s
-	$(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $<
+# This incantation ensures that changes to any subarch asm files will
+# force the corresponding object file to be rebuilt, even if the implicit
+# rule below goes indirectly through a .sub file.
+define mkasmdep
+$(dir $(patsubst %/,%,$(dir $(1))))$(notdir $(1:.s=.o)): $(1)
+endef
+$(foreach s,$(wildcard src/*/$(ARCH)*/*.s),$(eval $(call mkasmdep,$(s))))
+
+%.o: $(ARCH)$(ASMSUBARCH)/%.sub
+	$(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $(dir $<)$(shell cat $<)
 
 %.o: $(ARCH)/%.s
 	$(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $<
@@ -99,8 +107,8 @@ $(MEMOPS_SRCS:%.c=%.o) $(MEMOPS_SRCS:%.c=%.lo): CFLAGS += $(CFLAGS_MEMOPS)
 %.o: %.c $(GENH) $(IMPH)
 	$(CC) $(CFLAGS_ALL_STATIC) -c -o $@ $<
 
-%.lo: $(ARCH)$(ASMSUBARCH)/%.s
-	$(CC) $(CFLAGS_ALL_SHARED) -c -o $@ $<
+%.lo: $(ARCH)$(ASMSUBARCH)/%.sub
+	$(CC) $(CFLAGS_ALL_SHARED) -c -o $@ $(dir $<)$(shell cat $<)
 
 %.lo: $(ARCH)/%.s
 	$(CC) $(CFLAGS_ALL_SHARED) -c -o $@ $<