x86/boot: Micro-optimize GDT loading instructions

Rearrange the instructions a bit to use a 32-bit displacement once
instead of 2/3 times. This saves 8 bytes of machine code.

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Link: https://lore.kernel.org/r/20200202171353.3736319-8-nivedita@alum.mit.edu


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>