crypto/mem.c: switch to tsan_assist.h in CRYPTO_MDEBUG.

Rationale is that it wasn't providing accurate statistics anyway.
For statistics to be accurate CRYPTO_get_alloc_counts should acquire
a lock and lock-free additions should not be an option.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6786)