Don't strip object files on Cygwin

  Building for the Cygwin distro requires to be able to build debuginfo
  files.  This in turn requires to build object files without stripping.
  The stripping is performed by the next step after building which creates
  the debuginfo files.

Signed-off-by: Corinna Vinschen <vinschen@redhat.com>

Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>