Add internal/tsan_assist.h.

Goal here is to facilitate writing "thread-opportunistic" code that
withstands Thread Sanitizer's scrutiny. "Thread-opportunistic" is when
exact result is not required, e.g. some statistics, or execution flow
doesn't have to be unambiguous.

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