x86: board_r: Set the global data pointer after relocation

x86: board_r: Set the global data pointer after relocation

Since 'gd' is just a normal variable on 64-bit x86, it is relocated by the
time we get to board_init_r(). The old 'gd' variable is passed in as
parameter to board_init_r(), presumably for this situation.

Assign it on 64-bit x86 so that gd points to the correct data.

Options to improve this:
- Make gd a fixed register and remove the board_init_r() parameter
- Make all archs use this board_init_r() parameter

The second has a TODO in the code. The first has a TODO in a future commit
('x86: Support global_data on x86_64')

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>