arch/x86/core: Use CONFIG_QEMU_TARGET for QEMU specific code

Instead of both checking CONFIG_BOARD_QEMU_X86 and
CONFIG_BOARD_QEMU_X86_64, simply check for CONFIG_QEMU_TARGET.

This also helps qemu_x86_tiny, qemu_x86_lakemont and any
future/downstream QEMU targets to keep things simple.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>