x86: use data32 directive instead of macro for operand-size prefix

This file defines 'a32' and 'o32' macros to avoid magic numbers
of operand/address-size prefixing.

GAS supports 'data32' and 'addr32' for that purpose.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>