Refine the build system

The top-level 'Makefile' unconditionally includes mk/{arm,riscv}.mk, and
the shell substitutions inside those two files run immediately even when
the active architecture is Arm.

That is why the "no qemu-riscv32 found" message appears during an Arm
build.

Author: jserv

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
@@ -43,6 +41,7 @@ all: config bootstrap
 ifeq (,$(filter $(ARCH),$(ARCHS)))
 $(error Support ARM and RISC-V only. Select the target with "ARCH=arm" or "ARCH=riscv")
 endif
+include mk/$(ARCH).mk
 
 ifneq ("$(wildcard $(PWD)/config)","")
 TARGET_EXEC := $($(shell head -1 config | sed 's/.*: \([^ ]*\).*/\1/')_EXEC)