x86: add CODE_UNREACHABLE to z_x86_cpu_init

For some reason, unrelated code change triggered compiler
warning about this function returns even though it is
marked nonreturn. So add CODE_UNREACHABLE to silence
the warning, possibly to catch any errors.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>

arch/x86/core/intel64/cpu.c

@@ -208,4 +208,6 @@ FUNC_NORETURN void z_x86_cpu_init(struct x86_cpuboot *cpuboot)
 	/* Enter kernel, never return */
 	cpuboot->ready++;
 	cpuboot->fn(cpuboot->arg);
+
+	CODE_UNREACHABLE; /* LCOV_EXCL_LINE */
 }