ieee1275: Include a.out header in assembly of sparc64 boot loader

ieee1275: Include a.out header in assembly of sparc64 boot loader

Recent versions of binutils dropped support for the a.out and COFF
formats on sparc64 targets. Since the boot loader on sparc64 is
supposed to be an a.out binary and the a.out header entries are
rather simple to calculate in our case, we just write the header
ourselves instead of relying on external tools to do that.

Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>