Fix slow tls access after dlopen

[Paul Zimmermann]

       Hi Szabolcs,

please could you repost [1] as a distinct patch as requested by Carlos?

This issue affects the SageMath software tool, with a major slowdown:

https://trac.sagemath.org/ticket/34850

(see comment 97 which provides a short C program to reproduce the issue).

Best regards,
Paul

[1] https://patchwork.ozlabs.org/project/glibc/patch/b116855de71098ef7dd2875dd3237f8f3ecc12c2.1618301209.git.szabolcs.nagy@arm.com/