arm: set the relocated gd with gd->new_gd

Simplify the arm relocation behavior and get gd directly form new_gd,
as it is already done in crt0_64.S:

	ldr	x18, [x18, #GD_NEW_GD]		/* x18 <- gd->new_gd */

This patch avoid assumption on new GD location (new GD is below bd -
with #GD_SIZE offset).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>