build: add a "user" arch to allow the kernel to run as a program on the host machine

Makefile

@@ -1,6 +1,6 @@
 KERNEL_EXEC := socks_kernel
 
-ARCH := x86_64
+SOCKS_ARCH ?= x86_64
 
 MAKEFILE_PATH := $(abspath $(lastword $(MAKEFILE_LIST)))
 ROOT_DIR := $(patsubst %/,%,$(dir $(MAKEFILE_PATH)))
@@ -13,7 +13,7 @@ BUILD_DIR := build
 # Architecture-specific source files
 ####################################
 
-include arch/$(ARCH)/config.mk
+include arch/$(SOCKS_ARCH)/config.mk
 
 ####################################
 # Platform-independent kernel source files
@@ -31,10 +31,10 @@ LIBC_SRC_DIRS := stdio string ctype
 LIBC_C_FILES := $(foreach dir,$(LIBC_SRC_DIRS),$(wildcard libc/$(dir)/*.c))
 LIBC_OBJ := $(addprefix $(BUILD_DIR)/,$(LIBC_C_FILES:.c=.o))
 
-BUILD_ID := $(shell tools/generate_build_id.py --arch $(ARCH))
+BUILD_ID := $(shell tools/generate_build_id.py --arch $(SOCKS_ARCH))
 
 CFLAGS := $(CFLAGS) -DBUILD_ID=\"$(BUILD_ID)\" -g -Wall -Werror -pedantic \
-	-Iinclude -Iarch/$(ARCH)/include -Ilibc/include -Os
+	-Iinclude -Iarch/$(SOCKS_ARCH)/include -Ilibc/include -Os
 
 ASMFLAGS := $(ASMFLAGS) -DBUILD_ID=\"$(BUILD_ID)\"
 LDFLAGS := $(LDFLAGS) -g -Os
@@ -77,7 +77,7 @@ compile-db: $(BUILD_DIR)/compile_commands.json
 tools:
 	@$(MAKE) --no-print-directory -C tools
 
-include arch/$(ARCH)/extra.mk
+include arch/$(SOCKS_ARCH)/extra.mk
 
 .PHONY: all tools compile-db $(BUILD_DIR)/compile_commands.json