malloc: Initiate tcache shutdown even without allocations [BZ #28028]

After commit 1e26d351 ("malloc: Fix
tcache leak after thread destruction [BZ #22111]"),
tcache_shutting_down is still not early enough.  When we detach a
thread with no tcache allocated, tcache_shutting_down would still be
false.

Reviewed-by: DJ Delorie <dj@redhat.com>