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/lib/cpp/cxx/testcase.yaml

@@ -78,3 +78,11 @@ tests:
     build_only: true
     extra_configs:
       - CONFIG_STD_CPP23=y
+  # Verify struct _cpu_arch struct _thread_arch are compatible with C++ when
+  # specific configs are enabled.
+  cpp.main.empty_struct_size:
+    build_only: true
+    extra_configs:
+      - CONFIG_USERSPACE=n
+      - CONFIG_SMP=n
+      - CONFIG_FPU_SHARING=n