tls: Change some external symbols from Mbed TLS

In Mbed TLS:
    commit eccd88871767e2fba5f3a079cfdfcb77c376cf20
    Author: Gilles Peskine <Gilles.Peskine@arm.com>
    Date:   Tue Mar 10 12:19:08 2020 +0100

        Rename identifiers containing double-underscore

changes the name of a symbol we use.  As part of upgrading to newer
versions of Mbed TLS, change the name of the symbol we use.

A better fix would be to not use this symbol at all, and perhaps define
our own symbol the same way this internal symbol is defined within the
library.

Signed-off-by: David Brown <david.brown@linaro.org>