The wiki entry on Xen is pretty clear that a specific LDFLAG is required:
"Adds efi boot support, requires LDFLAG -melf_x86_64 for amd64"
...however, if you do NOT consult the wiki and DO NOT add that LDFLAG you will get "interesting" dom0 boot failures with screaming about double faults.
It would be nice if the required nature of the LDFLAG was either detected directly, or at least added as an ebuild comment for users to see. This cost me a few hours tracing the madness.
I'm trying to get Xen with efi support working, keep getting that double fault error. Adding LDFLAGS="-melf_x86_64" to make.conf did not help.