refactor Makefile and update CPU struct: rename stack pointer field for clarity

Makefile

@@ -1,32 +1,22 @@
-cc := gcc
-CFLAGS := -std=c11 -Wall -Wextra -Wpedantic -O2
-CFLAGS_DEBUG := -std=c11 -Wall -Wextra -Wpedantic -g -O0
+CC = gcc
+CFLAGS = -Wall -Wextra -O3 -Iinclude
+SRC_DIR = src
+OBJ_DIR = build
+BIN_DIR = bin
 
-SRC_DIR := src 
-BUILD_DIR := build 
-BIN_DIR := bin
-
-TARGET := $(BIN_DIR)/mos6502
-
-SRC := $(shell find  $(SRC_DIR) -name '*.c')
-OBJ := $(SRC:$(SRC_DIR)/%.c=$(BUILD_DIR)/%.o)
+SOURCES = $(wildcard $(SRC_DIR)/*.c)
+OBJECTS = $(SOURCES:$(SRC_DIR)/%.c=$(OBJ_DIR)/%.o)
+TARGET = $(BIN_DIR)/mos6502
 
 all: $(TARGET)
 
-$(TARGET): $(OBJ)
+$(TARGET): $(OBJECTS)
 	@mkdir -p $(BIN_DIR)
-	$(cc) $(OBJ) -o $(TARGET)
+	$(CC) $(OBJECTS) -o $@
 
-$(BUILD_DIR)/%.o: $(SRC_DIR)/%.c
-	@mkdir -p $(BUILD_DIR)
-	@mkdir -p $(dir $@)
-	$(cc) $(CFLAGS) -c $< -o $@
-
-debug: CFLAGS=$(CFLAGS_DEBUG)
-debug: clean $(TARGET)
+$(OBJ_DIR)/%.o: $(SRC_DIR)/%.c
+	@mkdir -p $(OBJ_DIR)
+	$(CC) $(CFLAGS) -c $< -o $@
 
 clean:
-	rm -rf $(BUILD_DIR) $(BIN_DIR)
-
-rebuild: clean all
-.PHONY: all clean rebuild debug
+	rm -rf $(OBJ_DIR) $(BIN_DIR)