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/alchemy/common/sleeper.S

@@ -95,7 +95,7 @@ LEAF(alchemy_sleep_au1000)
 
 	/* cache following instructions, as memory gets put to sleep */
 	la	t0, 1f
-	.set	mips3
+	.set	arch=r4000
 	cache	0x14, 0(t0)
 	cache	0x14, 32(t0)
 	cache	0x14, 64(t0)
@@ -121,7 +121,7 @@ LEAF(alchemy_sleep_au1550)
 
 	/* cache following instructions, as memory gets put to sleep */
 	la	t0, 1f
-	.set	mips3
+	.set	arch=r4000
 	cache	0x14, 0(t0)
 	cache	0x14, 32(t0)
 	cache	0x14, 64(t0)
@@ -163,7 +163,7 @@ LEAF(alchemy_sleep_au1300)
 	la	t1, 4f
 	subu	t2, t1, t0
 
-	.set	mips3
+	.set	arch=r4000
 
 1:	cache	0x14, 0(t0)
 	subu	t2, t2, 32