configure: enable cross-compilation of s390-ccw

configure: enable cross-compilation of s390-ccw

While container-based cross compilers are not supported, this already makes
it possible to build s390-ccw on any machine that has s390x GCC and binutils
installed.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220517092616.1272238-14-pbonzini@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220527153603.887929-25-alex.bennee@linaro.org>