MIPS: Fix gigaton of warning building with microMIPS.

With binutils 2.24 the attempt to switch with microMIPS mode to MIPS III
mode through .set mips3 results in *lots* of warnings like

{standard input}: Assembler messages:
{standard input}:397: Warning: the 64-bit MIPS architecture does not support the `smartmips' extension

during a kernel build.  Fixed by using .set arch=r4000 instead.

This breaks support for building the kernel with binutils 2.13 which
was supported for 32 bit kernels only anyway and 2.14 which was a bad
vintage for MIPS anyway.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/arch/mips/include/asm/r4kcache.h b/arch/mips/include/asm/r4kcache.h
index 69c2ada..ca64cbe 100644
--- a/arch/mips/include/asm/r4kcache.h
+++ b/arch/mips/include/asm/r4kcache.h
@@ -36,7 +36,7 @@
 	__asm__ __volatile__(						\
 	"	.set	push					\n"	\
 	"	.set	noreorder				\n"	\
-	"	.set	mips3\n\t				\n"	\
+	"	.set	arch=r4000				\n"	\
 	"	cache	%0, %1					\n"	\
 	"	.set	pop					\n"	\
 	:								\
@@ -204,7 +204,7 @@
 	__asm__ __volatile__(					\
 	"	.set	push			\n"		\
 	"	.set	noreorder		\n"		\
-	"	.set	mips3			\n"		\
+	"	.set	arch=r4000		\n"		\
 	"1:	cache	%0, (%1)		\n"		\
 	"2:	.set	pop			\n"		\
 	"	.section __ex_table,\"a\"	\n"		\