Windows: fix echo for nmake

It seems that nmake first tries to run executables on its own, and
only pass commands to cmd if that fails.  That means it's possible to
have nmake run something like 'echo.exe' when the builtin 'echo'
command was expected, which might give us unexpected results.

To get around this, we create our own echoing script and call it
explicitly from the nmake makefile.

Fixes #6670

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/6686)