Replace __gcov_flush with __gcov_dump

__gcov_flush was a private function and was removed from later GCC
versions (at least from 11.2.0, possibly earlier). Replace with the
documented public __gcov_dump.

src/V3Error.cpp

@@ -167,7 +167,7 @@ void V3Error::vlAbortOrExit() {
 }
 
 void V3Error::vlAbort() {
-    VL_GCOV_FLUSH();
+    VL_GCOV_DUMP();
     std::abort();
 }