target-arm: Clean up ENV_GET_CPU() usage

Commits ab1da857,
fdfba1a2,
2c17449b added usages of ENV_GET_CPU()
macro to target-specific code.

Use arm_env_get_cpu() instead and enforce separating variable
declarations.

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