MIPS: SB1: Fix excessive kernel warnings.

A kernel build with binutils 2.24 is going to emit warnings like

  CC      kernel/sys.o
{standard input}: Assembler messages:
{standard input}:701: Warning: the 32-bit MIPS architecture does not support the `mdmx' extension
{standard input}:701: Warning: the `mdmx' extension requires 64-bit FPRs
{standard input}:701: Warning: the `mips3d' extension requires MIPS32 revision 2 or greater
{standard input}:701: Warning: the `mips3d' extension requires 64-bit FPRs

for almost every file.  This is caused by changes to gas' interpretation
of .set semantics.  Fixed by explicitly disabling MIPS3D and MDMX for
Sibyte builds.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>