malloc: Don't use __libc_free for tcache cleanup

malloc: Don't use __libc_free for tcache cleanup

__libc_free must only be used for memory given out by __libc_malloc
and similar public apis, but tcache stores a cache of already freed
pointers and itself is allocated using internal malloc apis.  Strong
double free detection in __libc_free breaks tcache_thread_shutdown,
so use a cut down version of free to reset tcache entries.