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: NRalf Baechle <ralf@linux-mips.org>

arch/mips/Makefile

@@ -151,7 +151,7 @@ cflags-$(CONFIG_CPU_NEVADA)	+= $(call cc-option,-march=rm5200,-march=r5000) \
 			-Wa,--trap
 cflags-$(CONFIG_CPU_RM7000)	+= $(call cc-option,-march=rm7000,-march=r5000) \
 			-Wa,--trap
-cflags-$(CONFIG_CPU_SB1)	+= $(call cc-option,-march=sb1,-march=r5000) \
+cflags-$(CONFIG_CPU_SB1)	+= $(call cc-option,-march=sb1 -mno-mdmx -mno-mips3d,-march=r5000) \
 			-Wa,--trap
 cflags-$(CONFIG_CPU_R8000)	+= -march=r8000 -Wa,--trap
 cflags-$(CONFIG_CPU_R10000)	+= $(call cc-option,-march=r10000,-march=r8000) \