board/pc: use frame pointer unwinder in linux.config

The default ORC unwinder requires libelf to be built. While we could
build libelf using BR2_LINUX_KERNEL_NEEDS_HOST_LIBELF, we probably
don't really need the ORC unwinder in the PC defconfigs, so let's use
the frame pointer unwinder instead.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/123771107



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>