target-i386: Clean up ENV_GET_CPU() usage

Commits fdfba1a2,
f606604f and
2c17449b added usages of ENV_GET_CPU()
macro in target-specific code.

Use x86_env_get_cpu() or reuse existing X86CPU variable instead.

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