CONFIG_6xx is not available for thedefaultarchitecture x86.
Result is shown for architecture ppc

6xx/7xx/74xx/52xx/82xx/83xx

modulename: idle_6xx.ko

configname: CONFIG_6xx

Processor Type
└─>6xx/7xx/74xx/52xx/82xx/83xx
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
There are four types of PowerPC chips supported. The more common
types (601, 603, 604, 740, 750, 7400), the older Freescale
(formerly Motorola) embedded versions (821, 823, 850, 855, 860,
52xx, 82xx, 83xx), the IBM embedded versions (403 and 405) and
the Book E embedded processors from IBM (44x) and Freescale (85xx).
For support for 64-bit processors, set ARCH=powerpc.
Unless you are building a kernel for one of the embedded processor
systems, choose 6xx.
Also note that because the 52xx, 82xx, & 83xx family have a 603e
core, specific support for that chipset is asked later on.

source code: