roms/asmsrc: add back a "clean" target

It is too annoying to work on things in that directory without it.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

rv32/roms/asmsrc/Makefile

@@ -56,6 +56,10 @@ $(O)/%.s: $(O)/../%.asm $(HDRS)
 	mkdir -p $(@D) && cp -f $< $@
 
 clean:
+	find . -type f \
+		-name '*.o' -o -name '*.lst' -o -name '*.map' -o \
+		-name '*.s' -o -name '*.elf' -o -name '*.bin' \
+		| xargs rm -f
 
 spotless: clean
 	$(MAKE) O=*/* spotless_O