semihosting: Use env more often in do_common_semihosting

semihosting: Use env more often in do_common_semihosting

We've already loaded cs->env_ptr into a local variable; use it.
Since env is unconditionally used, we don't need a dummy use.

Reviewed-by: Luc Michel <lmichel@kalray.eu>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>