xen-{kernel/tools}: switch back to LLVM toolchain

Backport upstream fixes to correctly build Xen using LLVM 8
toolchain.

Note the fixes are not strictly needed for xen-tools since the Xen
shim build is only ever supposed to be booted using the PVH entry
point (and not the multiboot entry points). Yet given LLD8 erratic
behaviour with orphan section placement I think it's best to ensure
proper section placement for the shim also.

Sponsored by: Citrix Systems R&D
Reviewed by: mat
Differential revision: https://reviews.freebsd.org/D20816