Make OPENSSL_rdtsc universally available.

If such a timer/counter register is not available, the return value is always
zero.  This matches the assembly implementations' behaviour.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/5231)