CONFIG_SUN3X is not available for the selected architecture x86.
Result is shown for architecture m68k

Sun3x support

modulename: head.ko

configname: CONFIG_SUN3X

Linux Kernel Configuration
└─>Sun3x support
In linux kernel since version 2.6.12 )
This option enables support for the Sun 3x series of workstations.
Be warned that this support is very experimental.
Note that Sun 3x kernels are not compatible with Sun 3 hardware.
General Linux information on the Sun 3x series (now discontinued)
is at <http://www.angelfire.com/ca2/tech68k/sun3.html>.

If you don't want to compile a kernel for a Sun 3x, say N.
source code:
selects
CONFIG_LEGACY_TIMER_TICK
CONFIG_MMU_MOTOROLA
CONFIG_M68030


is selected by
CONFIG_SUN3X_ESP