configs/pc_x86_64_{bios,efi}_defconfig: use custom linux config

configs/pc_x86_64_{bios,efi}_defconfig: use custom linux config

Reusing the qemu x86-64 linux config isn't very obvious, so these defconfigs
aren't taken into consideration when the qemu defconfigs are updated,
breaking the build.

Instead use a custom linux config for the pc defconfigs.  With this, we also
can get rid of the fragment file containing the delta fra the qemu config.

Created by linux-update-defconfig (after turning of the fragment file).
Also drop the linux kernel version number from the file name as it just
causes extra noise whenever the kernel is bumped.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>