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
f16cc9660c