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>

arch/mips/kernel/idle.c

@@ -64,7 +64,7 @@ void r4k_wait_irqoff(void)
 	if (!need_resched())
 		__asm__(
 		"	.set	push		\n"
-		"	.set	mips3		\n"
+		"	.set	arch=r4000	\n"
 		"	wait			\n"
 		"	.set	pop		\n");
 	local_irq_enable();
@@ -82,7 +82,7 @@ static void rm7k_wait_irqoff(void)
 	if (!need_resched())
 		__asm__(
 		"	.set	push					\n"
-		"	.set	mips3					\n"
+		"	.set	arch=r4000				\n"
 		"	.set	noat					\n"
 		"	mfc0	$1, $12					\n"
 		"	sync						\n"
@@ -103,7 +103,7 @@ static void au1k_wait(void)
 	unsigned long c0status = read_c0_status() | 1;	/* irqs on */
 
 	__asm__(
-	"	.set	mips3			\n"
+	"	.set	arch=r4000			\n"
 	"	cache	0x14, 0(%0)		\n"
 	"	cache	0x14, 32(%0)		\n"
 	"	sync				\n"