bootloader: extlinux: Add extlinux-bootloader-gpt.

* gnu/bootloader/extlinux.scm (extlinux-bootloader-gpt): New exported
  variable.
 (install-extlinux)[mbr]: New argument.
 (install-extlinux-mbr, install-extlinux-gpt): New variables.
 (extlinux-bootloader)[installer]: Use install-extlinux-mbr.