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/io.h b/arch/mips/include/asm/io.h
index e221d1d..933b50e 100644
--- a/arch/mips/include/asm/io.h
+++ b/arch/mips/include/asm/io.h
@@ -331,7 +331,7 @@
 		if (irq)						\
 			local_irq_save(__flags);			\
 		__asm__ __volatile__(					\
-			".set	mips3"		"\t\t# __writeq""\n\t"	\
+			".set	arch=r4000"	"\t\t# __writeq""\n\t"	\
 			"dsll32 %L0, %L0, 0"			"\n\t"	\
 			"dsrl32 %L0, %L0, 0"			"\n\t"	\
 			"dsll32 %M0, %M0, 0"			"\n\t"	\
@@ -361,7 +361,7 @@
 		if (irq)						\
 			local_irq_save(__flags);			\
 		__asm__ __volatile__(					\
-			".set	mips3"		"\t\t# __readq" "\n\t"	\
+			".set	arch=r4000"	"\t\t# __readq" "\n\t"	\
 			"ld	%L0, %1"			"\n\t"	\
 			"dsra32 %M0, %L0, 0"			"\n\t"	\
 			"sll	%L0, %L0, 0"			"\n\t"	\