emulators/xen-kernel: use binutils to workaround LLD 8 bug

LLD 8 changed the behaviour regarding the placement of orphaned
sections, which produces a non-bootable Xen kernel:

https://bugs.llvm.org/show_bug.cgi?id=42327

Switch to GNU LD (and NM) until this is resolved.

Sponsored by: Citrix Systems R&D
Reviewed by: mat
Differential revision: https://reviews.freebsd.org/D20706