malloc: Keep some stats and dump them at process exit if LIBC_DUMP_MALLOC_STATS is set

Very bare-bones and barely useful. Can go away once the perf_event-based
malloc tracking is further along.

Libraries/LibC/stdlib.cpp

@@ -201,6 +201,10 @@ extern "C" {
 void exit(int status)
 {
     __cxa_finalize(nullptr);
+
+    if (getenv("LIBC_DUMP_MALLOC_STATS"))
+        serenity_dump_malloc_stats();
+
     extern void _fini();
     _fini();
     fflush(stdout);