[libc] Fix noreturn c++ compatibility

Stop using bare "noreturn" with library functions, and use the more
cross-compatible "_Noreturn" instead.

src/libraries/libc/__j6libc/assert_fail.cpp

@@ -1,7 +1,7 @@
 #include <assert.h>
 #include <stdlib.h>
 
-noreturn void
+void
 __assert_fail( const char *, const char *, unsigned, const char * )
 {
     // TODO: display message

src/libraries/libc/assert.h

@@ -20,7 +20,7 @@ extern "C" {
 #define assert( argument ) ( (void) 0 )
 #else
 
-noreturn void __assert_fail( const char *, const char *, unsigned, const char * );
+_Noreturn void __assert_fail( const char *, const char *, unsigned, const char * );
 
 #define assert( argument ) \
     do { if (!(argument)) { __assert_fail( #argument, __FILE__, __LINE__, __func__ ); }} while(0)

src/libraries/libc/stdnoreturn.h

@@ -10,10 +10,8 @@
   * file, You can obtain one at https://mozilla.org/MPL/2.0/.
   */
 
-#ifdef __cplusplus
-#define noreturn [[noreturn]]
-#elif __STDC_VERSION__ >= 201103L
+#if !defined(__cplusplus) && __STDC_VERSION__ >= 201103L
 #define noreturn _Noreturn
-#else
+#elif !defined(__cplusplus)
 #define noreturn
 #endif