trivial: Simplify the spots that use TARGET_BIG_ENDIAN as a numeric value

trivial: Simplify the spots that use TARGET_BIG_ENDIAN as a numeric value

TARGET_BIG_ENDIAN is *always* defined, either as 0 for little endian
targets or as 1 for big endian targets. So we can use this as a value
directly in places that need such a 0 or 1 for some reason, instead
of taking a detour through an additional local variable or something
similar.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>