emulators/virtualbox-ose-legacy: build fails on FreeBSD 12.4, 13.1 and 13.2 (all supported releases)

llvm 13 and 14 included in FreeBSD fails to compile, force llvm 15 on
all versions of the FreeBSD except lastest 13 and 14 where the llvm 15
is included.
Keep possibility to define different llvm via VBOX_LLVM_VER in make.conf.

PR: 265539
Reported by: freebsd@charles.lecklider.org
Tested by: grahamperrin
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D40408