Update jerry-libc unreachable asserts (#1379)

Newer compilers (especially clang) warn (and fail) on
`assert (!"message");` constructs typically used to assert on
unreachable code paths (multiple occurrences in jerry-libc). A more
up-to-date approach is to use `assert (false && "message");`. This
patch applies the pattern to all relevant asserts in jerry-libc.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-libc/target/posix/jerry-libc-target.c

@@ -134,13 +134,13 @@ fopen (const char *path, /**< file path */
       create_if_not_exist = true;
       if (mode[1] == '+')
       {
-        assert (!"unsupported mode a+");
+        assert (false && "unsupported mode a+");
       }
       break;
     }
     default:
     {
-      assert (!"unsupported mode");
+      assert (false && "unsupported mode");
     }
   }