qemu: Split kernel-x86_64.config

This attempts to make a "splitconfig" type of kernel configuration for
x86_64 so that we only include options that are intentionally enabled.
Other options will be set to default values by `make olddefconfig`.

Tested by building from scratch and booting to u-root shell using
qemu-system-x86_64.

Signed-off-by: David Hendricks <david.hendricks@gmail.com>

Author: dhendrix