cpu: remove support for x86

cpu/x86/Makefile

cpu/x86/Makefile.include

cpu/x86/include/cpu.h

cpu/x86/include/periph_cpu.h

cpu/x86/include/ucontext.h

cpu/x86/include/x86_cmos.h

cpu/x86/include/x86_interrupts.h

cpu/x86/include/x86_kernel_memory.h

cpu/x86/include/x86_memory.h

cpu/x86/include/x86_pci.h

cpu/x86/include/x86_pci_init.h

cpu/x86/include/x86_pci_strings.h

cpu/x86/include/x86_pic.h

cpu/x86/include/x86_pit.h

cpu/x86/include/x86_ports.h

cpu/x86/include/x86_reboot.h

cpu/x86/include/x86_registers.h

cpu/x86/include/x86_rtc.h

cpu/x86/include/x86_threading.h

cpu/x86/include/x86_uart.h