abtu: better ABTU_unreachable if __builtin_unreachable is unsupported

Some compilers such as Oracle Solaris C Compiler do not support
__builtin_unreachable().  If so, ABTU_unreachable() should tell the
compiler and the user that this line does not return.  This patch
implements better ABTU_unreachable() for such compilers.

src/include/abtu.h

@@ -112,18 +112,24 @@ static inline size_t ABTU_roundup_size(size_t val, size_t multiple)
 #define ABTU_unlikely(cond) (cond)
 #endif
 
-#ifdef HAVE___BUILTIN_UNREACHABLE
-#define ABTU_unreachable() __builtin_unreachable()
-#else
-#define ABTU_unreachable()
-#endif
-
 #ifdef HAVE_FUNC_ATTRIBUTE_NORETURN
 #define ABTU_noreturn __attribute__((noreturn))
 #else
 #define ABTU_noreturn
 #endif
 
+#ifdef HAVE___BUILTIN_UNREACHABLE
+#define ABTU_unreachable() __builtin_unreachable()
+#else
+/* abort is better than entering an unknown area.  First assert(0), which shows
+ * something if assert() is enabled.  If assert() is disabled, let's abort(). */
+static inline ABTU_noreturn void ABTU_unreachable(void)
+{
+    assert(0);
+    abort();
+}
+#endif
+
 #ifdef ABT_CONFIG_HAVE_ALIGNOF_GCC
 #define ABTU_alignof(type) (__alignof__(type))
 #elif defined(ABT_CONFIG_HAVE_ALIGNOF_C11)