Revert "[gcov] Delete flush_fn_list (unused since D83149)"

This reverts commit 01cdab0b33.

It was causing the instrprof-darwin-exports.c test to fail.
```
Undefined symbols for architecture x86_64:
  "_flush_fn_list", referenced from:
     -exported_symbol[s_list] command line option
```

compiler-rt/lib/profile/GCDAProfiling.c

@@ -127,6 +127,11 @@ struct fn_list {
  */
 struct fn_list writeout_fn_list;
 
+/*
+ *  A list of flush functions that our __gcov_flush() function should call, shared between all dynamic objects.
+ */
+struct fn_list flush_fn_list;
+
 /*
  *  A list of reset functions, shared between all dynamic objects.
  */