target-s390x: Clean up ENV_GET_CPU() usage

Commits f606604f,
2c17449b and
5ce5944d added usages of ENV_GET_CPU()
macro in target-specific code.

Use s390_env_get_cpu() instead.

Cc: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>