x86: Clean up asm-offsets

Move GD_BIST from lib/asm-offsets.c to arch/x86/lib/asm-offsets.c
as it is x86 arch specific stuff. Also remove GENERATED_GD_RELOC_OFF
which is not referenced anymore.

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