elf: Fix DTV gap reuse logic [BZ #27135]

elf: Fix DTV gap reuse logic [BZ #27135]

For some reason only dlopen failure caused dtv gaps to be reused.

It is possible that the intent was to never reuse modids for a
different module, but after dlopen failure all gaps are reused
not just the ones caused by the unfinished dlopened.

So the code has to handle reused modids already which seems to
work, however the data races at thread creation and tls access
(see bug 19329 and bug 27111) may be more severe if slots are
reused so this is scheduled after those fixes. I think fixing
the races are not simpler if reuse is disallowed and reuse has
other benefits, so set GL(dl_tls_dtv_gaps) whenever entries are
removed from the middle of the slotinfo list. The value does
not have to be correct: incorrect true value causes the next
modid query to do a slotinfo walk, incorrect false will leave
gaps and new entries are added at the end.

Fixes bug 27135.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>