x86, 386 removal: Document Nx586 as a 386 and thus unsupported

Per Alan Cox, Nx586 did not support WP in supervisor mode, making it a
386 by Linux kernel standards.  As such, it is too unsupported now.
Reported-by: NAlan Cox <alan@lxorguk.ukuu.org.uk>
Link: http://lkml.kernel.org/r/20121128205203.05868eab@pyramind.ukuu.org.ukSigned-off-by: NH. Peter Anvin <hpa@linux.intel.com>

arch/x86/Kconfig.cpu

@@ -15,7 +15,7 @@ config M486
 
 	  Note that the 386 is no longer supported, this includes
 	  AMD/Cyrix/Intel 386DX/DXL/SL/SLC/SX, Cyrix/TI 486DLC/DLC2,
-	  and UMC 486SX-S.
+	  UMC 486SX-S and the NexGen Nx586.
 
 	  The kernel will not necessarily run on earlier architectures than
 	  the one you have chosen, e.g. a Pentium optimized kernel will run on