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/asm.h b/arch/mips/include/asm/asm.h
index b153e79..7c26b28 100644
--- a/arch/mips/include/asm/asm.h
+++ b/arch/mips/include/asm/asm.h
@@ -146,7 +146,7 @@
 
 #define PREF(hint,addr)					\
 		.set	push;				\
-		.set	mips4;				\
+		.set	arch=r5000;			\
 		pref	hint, addr;			\
 		.set	pop
 
@@ -159,7 +159,7 @@
 
 #define PREFX(hint,addr)				\
 		.set	push;				\
-		.set	mips4;				\
+		.set	arch=r5000;			\
 		prefx	hint, addr;			\
 		.set	pop