[MIPS] Do not use -msym32 option for modules.

On 64-bit kernel, modules are loaded into XKSEG for now.  While XKSEG
address is not a sign-extended 32-bit address, we can not use -msym32
option.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>