allow reproducible builds of xen.gz

allow reproducible builds of xen.gz

The mkelf32 executable was using an uninitialized stack buffer for
padding after the ehdr and phdr are written to the xen file, which
leads to non-deterministic bytes in the binary and prevented Xen
hypervisors from being reproducibly built.

Additionally, the file was then compressed with gzip -9 without the
-n | --no-name flag, which lead to the xen.gz file having
non-deterministric bytes (the timestamp) in the compressed file.

Signed-off-by: Trammell Hudson <trammell.hudson@twosigma.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>