[build] Fix building on broken versions of GNU binutils

Some versions of GNU objcopy (observed with binutils 2.23.52.0.1 on
CentOS 7.0.1406) document the -D/--enable-deterministic-archives
option but fail to recognise the short form of the option.

Work around this problem by using the long form of the option.

Reported-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>