x86: Fix various code format issues in start16.S

Various minor code format issues are fixed in start16.S:
- U-boot -> U-Boot
- 32bit -> 32-bit
- Use TAB instead of SPACE to indent
- Move the indention location of the GDT comment block

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