gnu: Add support for aarch64's compiled kernel.

* gnu/packages/linux.scm (linux-libre)[arguments]: Add to the regex to
find and install the aarch64-linux kernel.
* gnu/system.scm (system-linux-image-file-name): Add entry for aarch64.

gnu/packages/linux.scm

@@ -340,7 +340,7 @@ for ARCH and optionally VARIANT, or #f if there is no such configuration."
                     (kmod   (assoc-ref (or native-inputs inputs) "kmod")))
                ;; Install kernel image, kernel configuration and link map.
                (for-each (lambda (file) (install-file file out))
-                         (find-files "." "^(\\.config|bzImage|zImage|vmlinuz|System\\.map)$"))
+                         (find-files "." "^(\\.config|bzImage|zImage|Image|vmlinuz|System\\.map)$"))
                ;; Install device tree files
                (for-each (lambda (file) (install-file file dtbdir))
                          (find-files "." "\\.dtb$"))

gnu/system.scm

@@ -392,6 +392,7 @@ from the initrd."
   (cond
    ((string-prefix? "arm" (%current-system)) "zImage")
    ((string-prefix? "mips" (%current-system)) "vmlinuz")
+   ((string-prefix? "aarch64" (%current-system)) "Image")
    (else "bzImage")))
 
 (define (operating-system-kernel-file os)