metag: Various other headers

Add the remaining metag header files:
 - byteorder.h, swab.h (byte order and swapping)
 - barrier.h, cpu.h. hwthread.h, processor.h (hardware thread related)
 - bug.h, elf.h, gpio.h, linkage.h, resource.h (other)

Signed-off-by: James Hogan <james.hogan@imgtec.com>