board_r: ARM[64] do not set gd again

For ARM / ARM64 the relocation routines already updated
gd to the new value. Don't set it again. This allows
compilation with clang as it cannot update gd directly.

cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>