config: move down companion libraries sub-menu

Move the companion libraries sub-menu down the main menu.
That way, the user does not need to go back and forth in the menu
to change options set by the different components that select the
companion libraries (binutils, gcc, gdb).

 # x86 specific options

 config ARCH_x86

     select ARCH_USE_MMU

     select ARCH_SUPPORT_ARCH

     select ARCH_SUPPORT_CPU

     select ARCH_SUPPORT_TUNE

     help

       The x86 architecture, as defined by:

         http://www.intel.com/