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 stage 2 compiler of shecc,
the build system didn't assign the variable correctly, causing the
bootstrapping process to execute the stage 1 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) \