um: allow SUBARCH=x86

nicked from patch by dwmw2 back in July
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/x86/um/Kconfig

@@ -15,8 +15,8 @@ config UML_X86
 	select GENERIC_FIND_FIRST_BIT
 
 config 64BIT
-	bool
-	default SUBARCH = "x86_64"
+	bool "64-bit kernel" if SUBARCH = "x86"
+	default SUBARCH != "i386"
 
 config X86_32
 	def_bool !64BIT