x86: Set up the global data pointer in C instead of asm

We currently assume that the global data pointer is at the start of
struct global_data. We want to remove this restriction, and it is
easiest to do this in C.

Remove the asm code and add equivalent code in C.

This idea was proposed by Graeme Russ here:
   http://patchwork.ozlabs.org/patch/199741/



Signed-off-by: Simon Glass <sjg@chromium.org>
[trini: Apply Graeme Russ' comments
http://patchwork.ozlabs.org/patch/206305/

 here, re-order]
Signed-off-by: Tom Rini <trini@ti.com>