sparc: fix tftpboot.img for sparc64 on little-endian host

piggyback_32 adapted to support sparc64:
- locating "HdrS" differs for sparc and sparc64
- sparc64 updates a_text, a_data + a_bss in the final a.out header

Updated Makefile to use piggyback_32 for sparc64.
Deleted the now unused piggyback_64.c

piggyback_32.c is host endian neutral and works on both
little-endian and big-endian hosts.
This fixes a long standing bug where sparc64 could not
generate tftpboot.img on a x86 host.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: David S. Miller <davem@davemloft.net>