Move DECLARE_GLOBAL_DATA_PTR to file scope

It can be optimised out by the compiler otherwise resulting
in obscure errors like a board not booting.

This has been documented in README since 2006 when these were
first fixed up for GCC 4.x.

Signed-off-by: John Rigby <john.rigby@linaro.org>

Fix some additional places.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Acked-By: Albert ARIBAUD <albert.aribaud@free.fr>