Move U64 macro from sha.h to sha512.c

Move U64 macro from sha.h to sha512.c

Summary:
U64 is too common name for macro, being in public header sha.h it
conflicts with other projects (WAVM in my case). Moving macro from
public header to the only .c file using it.

CLA: trivial

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/10579)