Fix the build system to use qemu-user as expected

Although the previous Makefile used the TARGET_EXEC variable to
store the name of an architecture-specific qemu executable, such as
qemu-arm or qemu-riscv32, for compiling the stage2 compiler of shecc,
the build system didn't assign the variable correctly, causing the
bootstrapping process to execute the stage1 compiler directly.

The above issue can be observed by running "make VERBOSE=1" to view
all build steps:

$ make VERBOSE=1
...
...
out/shecc-stage1.elf -o out/shecc-stage2.elf src/main.c
...

Therefore, these changes modify Makefile and .mk files to fix this
issue. After specifying the target architecture, the corresponding
.mk file now assigns the TARGET_EXEC variable directly, ensuring
the build system invokes qemu as expected.

Author: DrXiao

Makefile

@@ -17,8 +17,6 @@ CFLAGS := -O -g \
 BUILD_SESSION := .session.mk
 
 include mk/common.mk
-include mk/arm.mk
-include mk/riscv.mk
 -include $(BUILD_SESSION)
 
 STAGE0 := shecc
@@ -43,11 +41,7 @@ all: config bootstrap
 ifeq (,$(filter $(ARCH),$(ARCHS)))
 $(error Support ARM and RISC-V only. Select the target with "ARCH=arm" or "ARCH=riscv")
 endif
-
-ifneq ("$(wildcard $(PWD)/config)","")
-TARGET_EXEC := $($(shell head -1 config | sed 's/.*: \([^ ]*\).*/\1/')_EXEC)
-endif
-export TARGET_EXEC
+include mk/$(ARCH).mk
 
 config:
 	$(Q)ln -s $(PWD)/$(SRCDIR)/$(ARCH)-codegen.c $(SRCDIR)/codegen.c

mk/arm.mk

@@ -1,15 +1,15 @@
 ifeq ($(HOST_ARCH),armv7l) # detect ARMv7-A only and assume Linux-compatible
-    ARM_EXEC :=
+    TARGET_EXEC :=
 else
-    ARM_EXEC = qemu-arm
-    ARM_EXEC := $(shell which $(ARM_EXEC))
-    ifndef ARM_EXEC
+    TARGET_EXEC = qemu-arm
+    TARGET_EXEC := $(shell which $(TARGET_EXEC))
+    ifndef TARGET_EXEC
     $(warning "no qemu-arm found. Please check package installation")
     ARM_EXEC = echo WARN: unable to run
     endif
 endif
 
-export ARM_EXEC
+export TARGET_EXEC
 
 arm-specific-defs = \
     $(Q)$(PRINTF) \

mk/riscv.mk

@@ -1,11 +1,11 @@
-RISCV_EXEC = qemu-riscv32
-RISCV_EXEC := $(shell which $(RISCV_EXEC))
-ifndef RISCV_EXEC
+TARGET_EXEC = qemu-riscv32
+TARGET_EXEC := $(shell which $(TARGET_EXEC))
+ifndef TARGET_EXEC
 $(warning "no qemu-riscv32 found. Please check package installation")
-RISCV_EXEC = echo WARN: unable to run
+TARGET_EXEC = echo WARN: unable to run
 endif
 
-export RISCV_EXEC
+export TARGET_EXEC
 
 riscv-specific-defs = \
     $(Q)$(PRINTF) \