x86: simplify ljmp to 32-bit code

You can directly specify the label as the operand for ljmp.

This commit saves 4-byte code.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: fixed the gas warning]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>