elf: Support recursive use of dynamic TLS in interposed malloc

It turns out that quite a few applications use bundled mallocs that
have been built to use global-dynamic TLS (instead of the recommended
initial-exec TLS).  The previous workaround from
commit afe42e93 ("elf: Avoid some
free (NULL) calls in _dl_update_slotinfo") does not fix all
encountered cases unfortunatelly.

This change avoids the TLS generation update for recursive use
of TLS from a malloc that was called during a TLS update.  This
is possible because an interposed malloc has a fixed module ID and
TLS slot.  (It cannot be unloaded.)  If an initially-loaded module ID
is encountered in __tls_get_addr and the dynamic linker is already
in the middle of a TLS update, use the outdated DTV, thus avoiding
another call into malloc.  It's still necessary to update the
DTV to the most recent generation, to get out of the slow path,
which is why the check for recursion is needed.

The bookkeeping is done using a global counter instead of per-thread
flag because TLS access in the dynamic linker is tricky.

All this will go away once the dynamic linker stops using malloc
for TLS, likely as part of a change that pre-allocates all TLS
during pthread_create/dlopen.

Fixes commit d2123d68 ("elf: Fix slow
tls access after dlopen [BZ #19924]").

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>