vcl: WORDS_BIGENDIAN is never defined, use OSL_BIGENDIAN

Change-Id: I5186904b9703d6c9c49ca4c3b4a62f83423b1ae1
(cherry picked from commit 94d31d9ad1513504bb1415f0cfe70be380e18b9a)
Reviewed-on: https://gerrit.libreoffice.org/6612
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>