tests: riscv: Add test to verify C++ compilation

Using clang to build for RISC-V with these configs:

CONFIG_CPP=y
CONFIG_USERSPACE=n
CONFIG_SMP=n
CONFIG_FPU_SHARING=n

results in the following warnings:

include/zephyr/arch/riscv/structs.h:11:1: error: empty struct has size 0
in C, size 1 in C++ [-Werror,-Wextern-c-compat]
11 | struct _cpu_arch {
| ^

include/zephyr/arch/riscv/thread.h:68:1: error: empty struct has size 0
in C, size 1 in C++ [-Werror,-Wextern-c-compat]
68 | struct _thread_arch {
| ^

This commit adds a test with those configs to verify compilation
succeeds. Although gcc doesn't emit these warnings, we can test with gcc
as well with a BUILD_ASSERT in include/zephyr/arch/riscv/thread.h and
include/zephyr/arch/riscv/structs.h.

Signed-off-by: Tom Hughes <[email protected]>

Author: thughes

tests/arch/riscv/test_build_cpp/CMakeLists.txt

@@ -0,0 +1,9 @@
+# Copyright (c) 2025 Google Inc.
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(test_build_cpp)
+
+FILE(GLOB app_sources src/*.cpp)
+target_sources(app PRIVATE ${app_sources})

tests/arch/riscv/test_build_cpp/prj.conf

@@ -0,0 +1,4 @@
+# Copyright 2025 Google LLC
+# SPDX-License-Identifier: Apache-2.0
+
+CONFIG_CPP=y

tests/arch/riscv/test_build_cpp/src/main.cpp

@@ -0,0 +1,13 @@
+/*
+* Copyright (c) 2025 Google
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <zephyr/toolchain.h>
+
+BUILD_ASSERT(CONFIG_CPP, "C++ must be enabled");
+
+int main(void)
+{
+	return 0;
+}

tests/arch/riscv/test_build_cpp/testcase.yaml

@@ -0,0 +1,15 @@
+# Copyright (c) 2025 Google LLC
+# SPDX-License-Identifier: Apache-2.0
+
+tests:
+  # Verify struct _cpu_arch struct _thread_arch are compatible with C++
+  # when specific configs are enabled.
+  arch.riscv.test_build_cpp:
+    build_only: true
+    tags:
+      - riscv
+    arch_allow: riscv
+    extra_configs:
+      - CONFIG_USERSPACE=n
+      - CONFIG_SMP=n
+      - CONFIG_FPU_SHARING=n