ADD: decompressors: add boot-time XZ support
This implements the API defined in <linux/decompress/generic.h> which is
    used for kernel, initramfs, and initrd decompression.  This patch together
    with the first patch is enough for XZ-compressed initramfs and initrd;
    XZ-compressed kernel will need arch-specific changes.

    The buffering requirements described in decompress_unxz.c are stricter
    than with gzip, so the relevant changes should be done to the
    arch-specific code when adding support for XZ-compressed kernel.
    Similarly, the heap size in arch-specific pre-boot code may need to be
    increased (30 KiB is enough).

    The XZ decompressor needs memmove(), memeq() (memcmp() == 0), and
    memzero() (memset(ptr, 0, size)), which aren't available in all
    arch-specific pre-boot environments.  I'm including simple versions in
    decompress_unxz.c, but a cleaner solution would naturally be nicer.