Update README to clarify make arch-test usage

Running make arch-test without explicitly setting ENABLE_ARCH_TEST=1
may fail due to missing prebuilt binaries. This issue arises because
the default value of ENABLE_ARCH_TEST is 0, stopping Sail model from
being fetched.

Additionally, if ENABLE_SYSTEM=1 was previously set, the build system
fetches the kernel image instead of the Sail model. To ensure the
correct prebuilt binary is fetched, it is recommended to run make
distclean beforehand.

This serves as a temporary solution to sysprog21#575.

Author: vestata

README.md

@@ -69,7 +69,8 @@ $ brew install dtc
 ```
 
 #### Build and run system emulation
-Build and run using default images (the default images will be fetched from [rv32emu-prebuilt](https://github.com/sysprog21/rv32emu-prebuilt) before running):
+Build and run using default images (the default images will be fetched from [rv32emu-prebuilt](https://github.com/sysprog21/rv32emu-prebuilt) before running).
+If `ENABLE_ARCH_TEST=1` was previously set, run `make distclean` before proceeding.
 ```shell
 $ make ENABLE_SYSTEM=1 system
 ```
@@ -192,9 +193,10 @@ $ python3 -m pip install git+https://github.com/riscv/riscof
 
 [RISC-V GNU Compiler Toolchain](https://github.com/riscv-collab/riscv-gnu-toolchain) should be prepared in advance.
 You can obtain prebuilt GNU toolchain for `riscv32-elf` from the [Automated Nightly Release](https://github.com/riscv-collab/riscv-gnu-toolchain/releases).
+If `ENABLE_SYSTEM=1` was previously set, run `make distclean` before proceeding.
 Then, run the following command:
 ```shell
-$ make arch-test
+$ make ENABLE_ARCH_TEST=1 arch-test
 ```
 
 For macOS users, installing `sdiff` might be required:
@@ -204,7 +206,7 @@ $ brew install diffutils
 
 To run the tests for specific extension, set the environmental variable `RISCV_DEVICE` to one of `I`, `M`, `A`, `F`, `C`, `Zifencei`, `privilege`, `SYSTEM`.
 ```shell
-$ make arch-test RISCV_DEVICE=I
+$ make ENABLE_ARCH_TEST=1 arch-test RISCV_DEVICE=I
 ```
 
 Current progress of this emulator in riscv-arch-test (RV32):