Add rules to build LLVM IR binaries and libraries.

Running make libfoo.ll or make libfoo.bc within a library directory
will now give us an LLVM IR version of the library, and make foo.full.ll
or make foo.full.bc will give us an IR version of a binary.

As part of this change, we add an LLVM_LINK variable to sys.mk that can be
specified/overridden using an external toolchain.

Reviewed by: bdrewery, brooks
Approved by: rwatson (mentor)
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D8388