target-arm: Use global env in neon_helper.c helpers

Use the global 'env' variable in the helper functions in neon_helper.c.
This means we don't need to pass env as an argument to them any more.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>