emulators/virtualbox-ose: on 14 "Failed to construct device 'VMMDev' instance #0 (VERR_INTERNAL_ERROR_3)"

Force llvm from ports from 11 to 14 - by default it'll pick 14.
Use VBOX_LLVM_VER in make.conf to define different llvm.

PR: 270189
Reviewed by: cy, grahamperrin
Tested by: cy, grahamperrin, groenveld@acm.org, rkoberman@gmail.com
Approved by: arrowd
Differential Revision: https://reviews.freebsd.org/D40254