llvm: Reduce overlinking with the minimal llvm

We only need to link against libz and libzstd when linking against the
fill libllvm, libllvmminimal doesn't use either library. Move adding
libz and libzstd to the list of libraries to link against to where
we decide to use the full libllvm.

Reported by: Cristian Marussi <Cristian.Marussi@arm.com>
Reported by: Colin S. Gordon <csgordon@fastmail.com>
Reviewed by: dim
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42528

(cherry picked from commit 83771b1914e182c6324839e05ff82ee6f4a87c98)