[archiso] changes for syslinux 5.

These are some initial changes, while testing syslinux 5.

Some things with 5.01:

* whichsys.c32 and ifcpu64.c32 seems to be broken so, you need to enter manually at "boot:" prompt the right label, ie: boot: sys, boot: have64. (reported to upstream)
* PXE seems to be broken, after loading gpxelinux.0, does not found UI or DEFAULT directive configuration. (need more research)

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>