Makefile: start moving to macro based Makefile target generation

- libam-riscv32-nemu is working.
- libam on platforms other than riscv32-nemu has not been worked on yet.
- klib has not been worked on yet.

scripts/helpers/rules.mk

@@ -0,0 +1,47 @@
+
+# Usage:
+# $(1): Target prefix
+#
+# Generate build rules for files in variable <PREFIX>_SRCS. Use <PREFIX>_CFLAGS,
+# <PREFIX>_CXXFLAGS, <PREFIX>_ASFLAGS to control build flags.
+# E.g: 
+# $(eval $(call COMPILE_RULES,AM_))
+define COMPILE_RULES
+AM_OBJS := $(addprefix $(DST_DIR)/, $(addsuffix .o, $(AM_SRCS)))
+### Rule (compile): a single `.c` -> `.c.o` (gcc)
+$$(filter %.c.o, $$($(1)OBJS)): $$(DST_DIR)/%.c.o: %.c
+	@mkdir -p $$(dir $$@) && echo + CC $$<
+	@$$(CC) $$($(1)CFLAGS) -c -o $$@ $$(realpath $$<)
+
+### Rule (compile): a single `.cc` -> `.cc.o` (g++)
+$$(filter %.cc.o, $$($(1)OBJS)): $$(DST_DIR)/%.cc.o: %.cc
+	@mkdir -p $$(dir $$@) && echo + CXX $$<
+	@$$(CXX) $$($(1)CXXFLAGS) -c -o $$@ $$(realpath $$<)
+
+### Rule (compile): a single `.cpp` -> `.cpp.o` (g++)
+$$(filter %.cpp.o, $$($(1)OBJS)): $$(DST_DIR)/%.cpp.o: %.cpp
+	@mkdir -p $$(dir $$@) && echo + CXX $$<
+	@$$(CXX) $$($(1)CXXFLAGS) -c -o $$@ $$(realpath $$<)
+
+### Rule (compile): a single `.S` -> `.S.o` (gcc, which preprocesses and calls as)
+$$(filter %.S.o, $$($(1)OBJS)): $$(DST_DIR)/%.S.o: %.S
+	@mkdir -p $$(dir $$@) && echo + AS $$<
+	@$$(AS) $$($(1)ASFLAGS) -c -o $$@ $$(realpath $$<)
+
+### Rule (`#include` dependencies): paste in `.d` files generated by gcc on `-MMD`
+-include $$(addprefix $$(DST_DIR)/, $$(addsuffix .d, $$(filter %.c %.cc, $$(AM_SRCS))))
+endef
+
+# Usage:
+# $(1): Library name
+# $(2): Library variable prefix
+# E.g: 
+# $(eval $(call ADD_LIBRARY,am-riscv,AM_))
+define ADD_LIBRARY
+$(eval $(call COMPILE_RULES,$(2)))
+$(1): $$($(2)OBJS) 
+	@mkdir -p $$(dir $$@)
+	@echo + AR "->" $$(patsubst $$(CURDIR)/%,%,$(1))
+	@$$(AR) rcs $$@ $$($(2)OBJS)
+endef
+